[SPARK-10372] [CORE] basic test framework for entire spark scheduler

This is a basic framework for testing the entire scheduler.  The tests this adds aren't very interesting -- the point of this PR is just to setup the framework, to keep the initial change small, but it can be built upon to test more features (eg., speculation, killing tasks, blacklisting, etc.).

Author: Imran Rashid <irashid@cloudera.com>

Closes #8559 from squito/SPARK-10372-scheduler-integs.