[SPARK-11126][SQL] Fix the potential flaky test

The unit test added in #9132 is flaky. This is a follow up PR to add `listenerBus.waitUntilEmpty` to fix it.

Author: zsxwing <zsxwing@gmail.com>

Closes #9163 from zsxwing/SPARK-11126-follow-up.

sql/core/src/test/scala/org/apache/spark/sql/execution/ui/SQLListenerSuite.scala

@@ -316,10 +316,12 @@ class SQLListenerSuite extends SparkFunSuite with SharedSQLContext {
   test("SPARK-11126: no memory leak when running non SQL jobs") {
     val previousStageNumber = sqlContext.listener.stageIdToStageMetrics.size
     sqlContext.sparkContext.parallelize(1 to 10).foreach(i => ())
+    sqlContext.sparkContext.listenerBus.waitUntilEmpty(10000)
     // listener should ignore the non SQL stage
     assert(sqlContext.listener.stageIdToStageMetrics.size == previousStageNumber)
 
     sqlContext.sparkContext.parallelize(1 to 10).toDF().foreach(i => ())
+    sqlContext.sparkContext.listenerBus.waitUntilEmpty(10000)
     // listener should save the SQL stage
     assert(sqlContext.listener.stageIdToStageMetrics.size == previousStageNumber + 1)
   }