Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #5961 from tdas/SPARK-7430 and squashes the following commits:

d654978 [Tathagata Das] Fix scala style
fbf7174 [Tathagata Das] Added more verbose assert failure messages.
6aea07a [Tathagata Das] Ensure SynchronizedBuffer is used in every TestSuiteBase