YARN-9540. TestRMAppTransitions fails intermittently. Contributed by Tao Yang.

Author: abmodi

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/test/java/org/apache/hadoop/yarn/server/resourcemanager/rmapp/TestRMAppTransitions.java

@@ -653,6 +653,7 @@ protected RMApp testCreateAppFinished(
     RMAppEvent finishedEvent = new RMAppEvent(application.getApplicationId(),
         RMAppEventType.ATTEMPT_FINISHED, diagnostics);
     application.handle(finishedEvent);
+    rmDispatcher.await();
 
     //only run this verification if we created a finishing app
     if (submissionContext == null) {