[SPARK-23962][SQL][TEST] Fix race in currentExecutionIds().

SQLMetricsTestUtils.currentExecutionIds() was racing with the listener
bus, which lead to some flaky tests.  We should wait till the listener bus is
empty.

I tested by adding some Thread.sleep()s in SQLAppStatusListener, which
reproduced the exceptions I saw on Jenkins.  With this change, they went
away.

Author: Imran Rashid <irashid@cloudera.com>

Closes #21041 from squito/SPARK-23962.

sql/core/src/test/scala/org/apache/spark/sql/execution/metric/SQLMetricsTestUtils.scala

@@ -34,6 +34,7 @@ trait SQLMetricsTestUtils extends SQLTestUtils {
   import testImplicits._
 
   protected def currentExecutionIds(): Set[Long] = {
+    spark.sparkContext.listenerBus.waitUntilEmpty(10000)
     statusStore.executionsList.map(_.executionId).toSet
   }