testsuite: cover exec.service-override config

Problem: there is no test coverage that ensures a user cannot
specify an exec service override on a job unless explicitly allowed
by configuration.

Add such a test to t2412-sdexec-perilog.t.

t/t2412-sdexec-perilog.t

@@ -68,4 +68,9 @@ test_expect_success 'prolog was run under systemd' '
 test_expect_success 'epilog was run under systemd' '
 	grep "System epilog script" dmesg.out
 '
+test_expect_success 'configured exec service cannot be overridden' '
+	test_must_fail flux run \
+	    --setattr system.exec.bulkexec.service=rexec \
+	    /bin/true
+'
 test_done