testsuite: cover root job history

Problem: there is no test coverage for the job history plugin
storing a history for userid 0.

Add a test.

t/t2812-flux-job-last.t

@@ -4,6 +4,8 @@ test_description='Test the flux job last command'
 
 . $(dirname $0)/sharness.sh
 
+flux version | grep -q libflux-security && test_set_prereq FLUX_SECURITY
+
 test_under_flux 1
 
 test_expect_success 'flux-job last fails on invalid arguments' '
@@ -50,4 +52,22 @@ test_expect_success 'flux-job last does not list purged jobs' '
 	grep "job history is empty" nojob2.err
 '
 
+submit_as_root()
+{
+        FAKE_USERID=0
+        test_debug "echo running flux run $@ as userid $FAKE_USERID"
+        flux run --dry-run "$@" | \
+          flux python ${SHARNESS_TEST_SRCDIR}/scripts/sign-as.py $FAKE_USERID \
+            >job.signed
+        FLUX_HANDLE_USERID=$FAKE_USERID \
+          flux job submit --flags=signed job.signed
+}
+
+# issue #5475
+# Execution may fail but submission should work - enough for this test
+test_expect_success FLUX_SECURITY 'run a job as fake root' '
+	submit_as_root true &&
+	FLUX_HANDLE_USERID=0 flux job last
+'
+
 test_done