diff --git a/tests/testthat/_snaps/foreign.md b/tests/testthat/_snaps/foreign.md
new file mode 100644
index 0000000000..1f4d040c57
--- /dev/null
+++ b/tests/testthat/_snaps/foreign.md
@@ -0,0 +1,20 @@
+# reading graph in NCOL format
+
+    Code
+      read_graph(ncol_path, "ncol")
+    Output
+      IGRAPH UN-- 3 2 -- 
+      + attr: name (v/c)
+      + edges (vertex names):
+      [1] 0--1 1--2
+
+# reading graph in LGL format
+
+    Code
+      read_graph(lgl_path, "lgl")
+    Output
+      IGRAPH UN-- 3 2 -- 
+      + attr: name (v/c)
+      + edges (vertex names):
+      [1] 0--1 1--2
+
diff --git a/tests/testthat/helper.R b/tests/testthat/helper.R
index c0d7e6309c..03a7eabe58 100644
--- a/tests/testthat/helper.R
+++ b/tests/testthat/helper.R
@@ -20,8 +20,12 @@ local_rng_version <- function(version, .local_envir = parent.frame()) {
   orig
 }
 
-expect_isomorphic <- function(g1, g2) {
-  expect_true(isomorphic(g1, g2))
+expect_isomorphic <- function(g1, g2, ...) {
+  expect_true(isomorphic(g1, g2, ...))
+}
+
+expect_not_isomorphic <- function(g1, g2, ...) {
+  expect_false(isomorphic(g1, g2, ...))
 }
 
 expect_vcount <- function(graph, expected, ...) {
diff --git a/tests/testthat/test-components.R b/tests/testthat/test-components.R
index 50733ffd31..a30bde3338 100644
--- a/tests/testthat/test-components.R
+++ b/tests/testthat/test-components.R
@@ -111,7 +111,7 @@ test_that("largest strongly and weakly components are correct", {
     B - +C,
     C - +A
   )
-  expect_true(isomorphic(largest_component(g, "strong"), strongly))
+  expect_isomorphic(largest_component(g, "strong"), strongly)
 
   weakly <- graph_from_literal(
     A - +B,
@@ -119,13 +119,13 @@ test_that("largest strongly and weakly components are correct", {
     C - +A,
     C - +D
   )
-  expect_true(isomorphic(largest_component(g, "weak"), weakly))
+  expect_isomorphic(largest_component(g, "weak"), weakly)
 })
 
 test_that("the largest component of a null graph is a valid null graph", {
   nullgraph <- make_empty_graph(0)
 
-  expect_true(isomorphic(largest_component(make_empty_graph(0)), nullgraph))
+  expect_isomorphic(largest_component(make_empty_graph(0)), nullgraph)
 })
 
 test_that("articulation_points works", {
diff --git a/tests/testthat/test-conversion.R b/tests/testthat/test-conversion.R
index 93016bde07..f127975035 100644
--- a/tests/testthat/test-conversion.R
+++ b/tests/testthat/test-conversion.R
@@ -306,11 +306,11 @@ test_that("as_adj_list works", {
   expect_s3_class(adj_list[[1]], "igraph.vs")
   g_same <- graph_from_adj_list(adj_list, mode = "all")
   expect_isomorphic(g, g_same)
-  expect_true(isomorphic(g, g_same,
+  expect_isomorphic(g, g_same,
     method = "vf2",
     vertex.color1 = seq_len(vcount(g)),
     vertex.color2 = seq_len(vcount(g_same))
-  ))
+  )
 
   adj_el_list <- as_adj_edge_list(g)
   expect_s3_class(adj_el_list[[1]], "igraph.es")
@@ -345,11 +345,11 @@ test_that("as_adj_list works when return.vs.es is FALSE", {
   expect_s3_class(adj_list[[1]], NA)
   g2 <- graph_from_adj_list(adj_list, mode = "all")
   expect_isomorphic(g, g2)
-  expect_true(isomorphic(g, g2,
+  expect_isomorphic(g, g2,
     method = "vf2",
     vertex.color1 = 1:vcount(g),
     vertex.color2 = 1:vcount(g2)
-  ))
+  )
 
   adj_el_list <- as_adj_edge_list(g)
   for (i in seq_len(vcount(g))) {
diff --git a/tests/testthat/test-foreign.R b/tests/testthat/test-foreign.R
new file mode 100644
index 0000000000..31713d0d00
--- /dev/null
+++ b/tests/testthat/test-foreign.R
@@ -0,0 +1,40 @@
+test_that("writing Pajek files works", {
+  g <- make_ring(9)
+  V(g)$color <- rep_len(c("red", "green", "yellow"), length.out = 9)
+
+  tc <- withr::local_connection(rawConnection(raw(0), "w"))
+
+  write_graph(g, format = "pajek", file = tc)
+
+  expect_equal(
+    rawToChar(rawConnectionValue(tc)),
+    "*Vertices 9\n1 \"1\" ic \"red\"\n2 \"2\" ic \"green\"\n3 \"3\" ic \"yellow\"\n4 \"4\" ic \"red\"\n5 \"5\" ic \"green\"\n6 \"6\" ic \"yellow\"\n7 \"7\" ic \"red\"\n8 \"8\" ic \"green\"\n9 \"9\" ic \"yellow\"\n*Edges\n1 2\n2 3\n3 4\n4 5\n5 6\n6 7\n7 8\n8 9\n1 9\n"
+  )
+})
+
+test_that("reading GraphML file works", {
+  skip_if_no_graphml()
+
+  g <- read_graph(f <- gzfile("zachary.graphml.gz"), format = "graphml")
+  g2 <- make_graph("zachary")
+
+  expect_isomorphic(g2, g)
+})
+
+test_that("reading graph in NCOL format", {
+  local_igraph_options(print.id = FALSE)
+
+  ncol_path <- withr::local_tempfile(pattern = "testfile", fileext = ".ncol")
+  g <- make_graph(c(1, 2, 2, 3))
+  write_graph(g, ncol_path, "ncol")
+  expect_snapshot(read_graph(ncol_path, "ncol"))
+})
+
+test_that("reading graph in LGL format", {
+  local_igraph_options(print.id = FALSE)
+
+  lgl_path <- withr::local_tempfile(pattern = "testfile", fileext = ".lgl")
+  g <- make_graph(c(1, 2, 2, 3))
+  write_graph(g, lgl_path, "lgl")
+  expect_snapshot(read_graph(lgl_path, "lgl"))
+})
diff --git a/tests/testthat/test-isomorphism.R b/tests/testthat/test-isomorphism.R
index cbb2b6cf11..4fb497ccfa 100644
--- a/tests/testthat/test-isomorphism.R
+++ b/tests/testthat/test-isomorphism.R
@@ -1,15 +1,15 @@
 test_that("isomorphic", {
   g <- graph_from_literal(A - B - C - A)
-  expect_true(isomorphic(g, g))
-  expect_true(isomorphic(g, g, method = "direct"))
-  expect_true(isomorphic(g, g, method = "vf2"))
-  expect_true(isomorphic(g, g, method = "bliss"))
+  expect_isomorphic(g, g)
+  expect_isomorphic(g, g, method = "direct")
+  expect_isomorphic(g, g, method = "vf2")
+  expect_isomorphic(g, g, method = "bliss")
 
   g2 <- graph_from_literal(A - B - C)
-  expect_false(isomorphic(g, g2))
-  expect_false(isomorphic(g, g2, method = "direct"))
-  expect_false(isomorphic(g, g2, method = "vf2"))
-  expect_false(isomorphic(g, g2, method = "bliss"))
+  expect_not_isomorphic(g, g2)
+  expect_not_isomorphic(g, g2, method = "direct")
+  expect_not_isomorphic(g, g2, method = "vf2")
+  expect_not_isomorphic(g, g2, method = "bliss")
 })
 
 test_that("subgraph_isomorphic", {
diff --git a/tests/testthat/test-operators.R b/tests/testthat/test-operators.R
index 28a7020b5a..6821d69b23 100644
--- a/tests/testthat/test-operators.R
+++ b/tests/testthat/test-operators.R
@@ -87,7 +87,7 @@ test_that("edge reversal works", {
   directed_graph <- make_graph(~ 1 -+ 2, 1 -+ 3, 1 -+ 4, 2 -+ 3, 3 -+ 4)
   reverse_directed_graph <- reverse_edges(directed_graph, 1:3)
   expected <- make_graph(~ 1 +- 2, 1 +- 3, 1 +- 4, 2 -+ 3, 3 -+ 4)
-  expect_true(isomorphic(reverse_directed_graph, expected))
+  expect_isomorphic(reverse_directed_graph, expected)
 
   reverse_all_directed_graph <- reverse_edges(directed_graph)
   expect_vcount(reverse_all_directed_graph, vcount(directed_graph))
diff --git a/tests/testthat/test-pajek.R b/tests/testthat/test-pajek.R
deleted file mode 100644
index 8a44c41f97..0000000000
--- a/tests/testthat/test-pajek.R
+++ /dev/null
@@ -1,11 +0,0 @@
-test_that("writing Pajek files works", {
-  g <- make_ring(9)
-  V(g)$color <- rep(c("red", "green", "yellow"), 3)
-
-  tc <- rawConnection(raw(0), "w")
-  write_graph(g, format = "pajek", file = tc)
-  out <- rawToChar(rawConnectionValue(tc))
-  close(tc)
-
-  expect_equal(out, "*Vertices 9\n1 \"1\" ic \"red\"\n2 \"2\" ic \"green\"\n3 \"3\" ic \"yellow\"\n4 \"4\" ic \"red\"\n5 \"5\" ic \"green\"\n6 \"6\" ic \"yellow\"\n7 \"7\" ic \"red\"\n8 \"8\" ic \"green\"\n9 \"9\" ic \"yellow\"\n*Edges\n1 2\n2 3\n3 4\n4 5\n5 6\n6 7\n7 8\n8 9\n1 9\n")
-})
diff --git a/tests/testthat/test-read_graph.R b/tests/testthat/test-read_graph.R
deleted file mode 100644
index 2f671f872e..0000000000
--- a/tests/testthat/test-read_graph.R
+++ /dev/null
@@ -1,22 +0,0 @@
-test_that("reading GraphML file works", {
-  skip_if_no_graphml()
-
-  g <- read_graph(f <- gzfile("zachary.graphml.gz"), format = "graphml")
-  g2 <- make_graph("zachary")
-
-  expect_true(isomorphic(g2, g))
-})
-
-test_that("reading graph in NCOL format", {
-  ncol_path <- tempfile("testfile", fileext = ".ncol")
-  g <- make_graph(c(1, 2, 2, 3))
-  write_graph(g, ncol_path, "ncol")
-  expect_no_error(g2 <- read_graph(ncol_path, "ncol"))
-})
-
-test_that("reading graph in LGL format", {
-  lgl_path <- tempfile("testfile", fileext = ".lgl")
-  g <- make_graph(c(1, 2, 2, 3))
-  write_graph(g, lgl_path, "lgl")
-  expect_no_error(g2 <- read_graph(lgl_path, "lgl"))
-})
diff --git a/tests/testthat/test-trees.R b/tests/testthat/test-trees.R
index 1dba898ba5..b02ee6a4d7 100644
--- a/tests/testthat/test-trees.R
+++ b/tests/testthat/test-trees.R
@@ -73,26 +73,26 @@ test_that("to_prufer and make_from_prufer works for trees", {
   g <- make_tree(13, 3, mode = "undirected")
   seq <- to_prufer(g)
   g2 <- make_from_prufer(seq)
-  expect_true(isomorphic(g, g2))
+  expect_isomorphic(g, g2)
 
   g <- make_tree(13, 3, mode = "out")
   seq <- to_prufer(g)
   g2 <- make_from_prufer(seq)
   g3 <- as_undirected(g)
-  expect_true(isomorphic(g2, g3))
+  expect_isomorphic(g2, g3)
 
   g <- make_tree(13, 3, mode = "in")
   seq <- to_prufer(g)
   g2 <- make_from_prufer(seq)
   g3 <- as_undirected(g)
-  expect_true(isomorphic(g2, g3))
+  expect_isomorphic(g2, g3)
 })
 
 test_that("make_(from_prufer(...)) works", {
   g <- make_tree(13, 3, mode = "undirected")
   seq <- to_prufer(g)
   g2 <- make_(from_prufer(seq))
-  expect_true(isomorphic(g, g2))
+  expect_isomorphic(g, g2)
 })
 
 test_that("to_prufer prints an error for non-trees", {