YARN-5521. Fix random failure of TestCapacityScheduler#testKillAllAppsInQueue (sandflee via Varun Saxena)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/test/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/TestCapacityScheduler.java

@@ -2184,6 +2184,7 @@ public class TestCapacityScheduler {
 
     // check postconditions
     rm.waitForState(app.getApplicationId(), RMAppState.KILLED);
+    rm.waitForAppRemovedFromScheduler(app.getApplicationId());
     appsInRoot = scheduler.getAppsInQueue("root");
     assertTrue(appsInRoot.isEmpty());