YARN-5318. Fix intermittent test failure of TestRMAdminService#testRefreshNodesResourceWithFileSystemBasedConfigurationProvider. (Jun Gong via Varun Saxena).

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

@@ -232,6 +232,7 @@ public class TestRMAdminService {
 
     rm.adminService.refreshNodesResources(
         RefreshNodesResourcesRequest.newInstance());
+    rm.drainEvents();
 
     RMNode niAfter = rm.getRMContext().getRMNodes().get(nid);
     Resource resourceAfter = niAfter.getTotalCapability();