[SPARK-26732][CORE][TEST] Wait for listener bus to process events in SparkContextInfoSuite.

Otherwise the RDD data may be out of date by the time the test tries to check it.

Tested with an artificial delay inserted in AppStatusListener.

Closes #23654 from vanzin/SPARK-26732.

Authored-by: Marcelo Vanzin <vanzin@cloudera.com>
Signed-off-by: Takeshi Yamamuro <yamamuro@apache.org>
(cherry picked from commit 6a2f3dc)
Signed-off-by: Takeshi Yamamuro <yamamuro@apache.org>

Author: Marcelo Vanzin

core/src/test/scala/org/apache/spark/SparkContextInfoSuite.scala

@@ -60,6 +60,7 @@ class SparkContextInfoSuite extends SparkFunSuite with LocalSparkContext {
     val rdd = sc.makeRDD(Array(1, 2, 3, 4), 2).cache()
     assert(sc.getRDDStorageInfo.size === 0)
     rdd.collect()
+    sc.listenerBus.waitUntilEmpty(10000)
     assert(sc.getRDDStorageInfo.size === 1)
     assert(sc.getRDDStorageInfo.head.isCached)
     assert(sc.getRDDStorageInfo.head.memSize > 0)