[Hot Fix #469] Fix flaky test in SparkListenerSuite

The two modified tests may fail if the race condition does not bid in our favor...

Author: Andrew Or <andrewor14@gmail.com>

Closes #516 from andrewor14/stage-info-test-fix and squashes the following commits:

b4b6100 [Andrew Or] Add/replace missing waitUntilEmpty() calls to listener bus