[SPARK-13405][STREAMING][TESTS] Make sure no messages leak to the next test

## What changes were proposed in this pull request?

Fixed the test failure `org.apache.spark.sql.util.ContinuousQueryListenerSuite.event ordering`: https://amplab.cs.berkeley.edu/jenkins/job/spark-master-test-maven-hadoop-2.6/202/testReport/junit/org.apache.spark.sql.util/ContinuousQueryListenerSuite/event_ordering/

```
      org.scalatest.exceptions.TestFailedException:
Assert failed: : null equaled null onQueryTerminated called before onQueryStarted
org.scalatest.Assertions$class.newAssertionFailedException(Assertions.scala:500)
	org.scalatest.FunSuite.newAssertionFailedException(FunSuite.scala:1555)
	org.scalatest.Assertions$AssertionsHelper.macroAssert(Assertions.scala:466)
	org.apache.spark.sql.util.ContinuousQueryListenerSuite$QueryStatusCollector$$anonfun$onQueryTerminated$1.apply$mcV$sp(ContinuousQueryListenerSuite.scala:204)
	org.scalatest.concurrent.AsyncAssertions$Waiter.apply(AsyncAssertions.scala:349)
	org.apache.spark.sql.util.ContinuousQueryListenerSuite$QueryStatusCollector.onQueryTerminated(ContinuousQueryListenerSuite.scala:203)
	org.apache.spark.sql.execution.streaming.ContinuousQueryListenerBus.doPostEvent(ContinuousQueryListenerBus.scala:67)
	org.apache.spark.sql.execution.streaming.ContinuousQueryListenerBus.doPostEvent(ContinuousQueryListenerBus.scala:32)
	org.apache.spark.util.ListenerBus$class.postToAll(ListenerBus.scala:63)
	org.apache.spark.sql.execution.streaming.ContinuousQueryListenerBus.postToAll(ContinuousQueryListenerBus.scala:32)
```

In the previous codes, when the test `adding and removing listener` finishes, there may be still some QueryTerminated events in the listener bus queue. Then when `event ordering` starts to run, it may see these events and throw the above exception.

This PR just added `waitUntilEmpty` in `after` to make sure all events be consumed after each test.

## How was the this patch tested?

Jenkins tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #11275 from zsxwing/SPARK-13405.