Adjust the regex of spawn-task-external-bad test

Now the program fails in runtime directly as the stream is not
initialized when Nanos6 attempts to write events.

test/rt/nanos6/CMakeLists.txt

@@ -51,7 +51,7 @@ nanos6_rt_test(taskloop.c)
 nanos6_rt_test(spawn-task.c)
 nanos6_rt_test(spawn-task-external.c)
 nanos6_rt_test(spawn-task-external-bad.c SHOULD_FAIL
-  REGEX "ovni_finish: thread [0-9]* is not dead")
+  REGEX "ovni_ev_add: thread is not initialized")
 
 # Test multiple instrumentation levels
 nanos6_rt_test(simple-task.c NAME simple-task-level-1 LEVEL 1)

test/rt/nanos6/spawn-task-external-bad.c

@@ -52,12 +52,17 @@ complete_func(void *arg)
 	complete = 1;
 }
 
-/* Call the nanos6_spawn_function from an external thread */
+/* Call the nanos6_spawn_function from an external thread without
+ * instrumentation. */
 static void *
 spawn(void *arg)
 {
+	/* Here Nanos6 will try to write events in the thread stream,
+	 * but the thread has not been initialized so it will abort */
 	nanos6_spawn_function(polling_func, arg,
 			complete_func, NULL, "polling_task");
+
+	/* Not reached */
 	return NULL;
 }