[SPARK-26079][SQL] Ensure listener event delivery in StreamingQueryLi…

…stenersConfSuite.

Events are dispatched on a separate thread, so need to wait for them to be
actually delivered before checking that the listener got them.

Closes apache#23050 from vanzin/SPARK-26079.

Authored-by: Marcelo Vanzin <vanzin@cloudera.com>
Signed-off-by: hyukjinkwon <gurwls223@apache.org>

sql/core/src/test/scala/org/apache/spark/sql/streaming/StreamingQueryListenersConfSuite.scala

@@ -30,7 +30,6 @@ class StreamingQueryListenersConfSuite extends StreamTest with BeforeAndAfter {
 
   import testImplicits._
 
-
   override protected def sparkConf: SparkConf =
     super.sparkConf.set("spark.sql.streaming.streamingQueryListeners",
       "org.apache.spark.sql.streaming.TestListener")
@@ -41,6 +40,8 @@ class StreamingQueryListenersConfSuite extends StreamTest with BeforeAndAfter {
       StopStream
     )
 
+    spark.sparkContext.listenerBus.waitUntilEmpty(5000)
+
     assert(TestListener.queryStartedEvent != null)
     assert(TestListener.queryTerminatedEvent != null)
   }