YARN-9798. ApplicationMasterServiceTestBase#testRepeatedFinishApplicationMaster fails intermittently. Contributed by Tao Yang.

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

@@ -382,6 +382,7 @@ public abstract class ApplicationMasterServiceTestBase {
       for (int i = 0; i < 10; i++) {
         am1.unregisterAppAttempt(req, false);
       }
+      rm.drainEvents();
       Assert.assertEquals("Expecting only one event", 1,
           dispatcher.getEventCount());
     } finally {