testsuite: verify error message on duplicate jobtap plugin load

Problem: The test for expected failure when loading a duplication
jobtap plugin does not validate that the correct error message
is emitted.

Validate the error message in addition to ensuring that the plugin
load fails.

t/t2212-job-manager-plugins.t

@@ -50,7 +50,9 @@ test_expect_success 'job-manager: multiple plugins can be loaded' '
 '
 test_expect_success 'job-manager: loading duplicate plugins fails' '
 	flux jobtap load ${PLUGINPATH}/args.so &&
-	test_must_fail flux jobtap load ${PLUGINPATH}/args.so &&
+	test_must_fail flux jobtap load ${PLUGINPATH}/args.so >dup.log 2>&1 &&
+	test_debug "cat dup.log" &&
+	grep "already loaded" dup.log &&
 	flux jobtap remove args.so
 '
 test_expect_success 'job-manager: query of plugin works' '