YARN-5362. TestRMRestart#testFinishedAppRemovalAfterRMRestart can fail. Contributed by sandflee.

(cherry picked from commit d6d41e820ac7b3ba73f5e4ea1ee72715dc1ffc9f)

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

@@ -1585,6 +1585,7 @@ public class TestRMRestart extends ParameterizedSchedulerTestBase {
     RMApp app1 = rm2.submitApp(200);
     MockAM am1 = launchAM(app1, rm2, nm1);
     finishApplicationMaster(app1, rm2, nm1, am1);
+    rm2.drainEvents();
 
     // the first app0 get kicked out from both rmContext and state store
     Assert.assertNull(rm2.getRMContext().getRMApps()