Add a test illustrating the issue

tests/test-dirs/locate/exception-at-origin-issue.t

@@ -0,0 +1,32 @@
+  $ cat >dune-project <<EOF
+  > (lang dune 3.0)
+  > EOF
+
+  $ cat >dune <<EOF
+  > (library
+  >  (name foo))
+  > EOF
+
+  $ cat >foo.ml <<EOF
+  > exception Unix_error = Unix.Unix_error
+  > EOF
+
+  $ dune build 
+
+FIXME: Merlin should jump to stdlib Unix.Unix_error
+  $ $MERLIN single locate -look-for ml -position 1:31 \
+  > -filename foo.ml <foo.ml
+  {
+    "class": "return",
+    "value": "Already at definition point",
+    "notifications": []
+  }
+
+FIXME: Merlin should jump to stdlib Unix
+  $ $MERLIN single locate -look-for ml -position 1:25 \
+  > -filename foo.ml <foo.ml
+  {
+    "class": "return",
+    "value": "Already at definition point",
+    "notifications": []
+  }