testsuite: add system test for flux exec --jobid

Problem: there is no system test coverage for flux exec --jobid.

Add a system test script.

Author: garlick

t/Makefile.am

@@ -320,6 +320,7 @@ dist_check_SCRIPTS = \
 	system/0001-basic.t \
 	system/0002-exec-with-imp.t \
 	system/0004-recovery.t \
+	system/0005-exec.t \
 	issues/t0441-kvs-put-get.sh \
 	issues/t0505-msg-handler-reg.lua \
 	issues/t0821-kvs-segfault.sh \

t/system/0005-exec.t

@@ -0,0 +1,23 @@
+#
+#  Check exec service with guest/owner
+#
+
+test_expect_success 'start a long-running guest job' '
+        flux submit -n1 --wait-event=start sleep inf &&
+	jobid=$(flux job last)
+'
+test_expect_success 'flux exec --jobid fails as guest' '
+	test_must_fail flux exec --jobid=$jobid /bin/true
+'
+test_expect_success 'flux exec --jobid fails as instance owner' '
+	test_must_fail sudo -u flux flux exec --jobid=$jobid /bin/true
+'
+test_expect_success 'flux exec without --jobid works as instance owner' '
+	sudo -u flux flux exec -r 0 /bin/true
+'
+test_expect_success 'flux exec without --jobid fails as guest' '
+	test_must_fail flux exec -r 0 /bin/true
+'
+test_expect_success 'cancel long-running job' '
+	flux cancel $jobid
+'