ZEPPELIN-3277. NotebookServerTest.testMakeSureNoAngularObjectBroadcas…

…tToWebsocketWhoFireTheEvent fails

### What is this PR for?

Trivial fix for flaky test, see the comment for details.

### What type of PR is it?
[Bug Fix]

### Todos
* [ ] - Task

### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-3277

### How should this be tested?
* CI pass

### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No

Author: Jeff Zhang <zjffdu@apache.org>

Closes apache#2838 from zjffdu/ZEPPELIN-3277 and squashes the following commits:

68827d7 [Jeff Zhang] ZEPPELIN-3277. NotebookServerTest.testMakeSureNoAngularObjectBroadcastToWebsocketWhoFireTheEvent fails

zeppelin-server/src/test/java/org/apache/zeppelin/socket/NotebookServerTest.java

@@ -123,6 +123,8 @@ public void testMakeSureNoAngularObjectBroadcastToWebsocketWhoFireTheEvent() thr
       }
       Thread.sleep(100);
     }
+    // sleep for 1 second to make sure job running thread finish to fire event. See ZEPPELIN-3277
+    Thread.sleep(1000);
 
     // add angularObject
     interpreterGroup.getAngularObjectRegistry().add("object1", "value1", note1.getId(), null);