[SPARK-25805][SQL][TEST] Fix test for SPARK-25159

The original test would sometimes fail if the listener bus did not keep
up, so just wait till the listener bus is empty.  Tested by adding a
sleep in the listener, which made the test consistently fail without the
fix, but pass consistently after the fix.

Closes #22799 from squito/SPARK-25805.

Authored-by: Imran Rashid <irashid@cloudera.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: squito

sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala

@@ -2537,6 +2537,7 @@ class DataFrameSuite extends QueryTest with SharedSQLContext {
 
       val df = spark.read.json(path.getCanonicalPath)
       assert(df.columns === Array("i", "p"))
+      spark.sparkContext.listenerBus.waitUntilEmpty(10000)
       assert(numJobs == 1)
     }
   }