testsuite: add issue reproducer for flux-framework#5518

Problem: In issue flux-framework#5518 the job validator worker process becomes hung
because a write failure terminates the process without cleaning up
pending futures.

Add a test that reproduces the error and ensure that failed job
submissions do not hang and that future submissions work after the
errors have cleared.

t/issues/t5518-job-validator-hang.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+# 
+# Run an instance with a very small job-ingest working buffer size
+# and ensure the worker does not hang after errors are returned
+#
+id=$(flux alloc -n1 --bg --conf=ingest.buffer-size=4k)
+printf "Launched single core alloc job $id\n"
+
+# Submission of more than 1 job should have some failures, but should not
+# hang:
+flux proxy $id flux submit --cc=1-5 --watch hostname
+rc=$?
+printf "submission of multiple jobs got rc=$rc\n"
+test $rc -ne 0 || exit 1
+
+# Another small job should succeed:
+flux proxy $id flux run --env=-* --env=PATH hostname || exit 1
+printf "submission of single job still works\n"