[SPARK-1143] Separate pool tests into their own suite.

The current TaskSchedulerImplSuite includes some tests that are
actually for the TaskSchedulerImpl, but the remainder of the tests avoid using
the TaskSchedulerImpl entirely, and actually test the pool and scheduling
algorithm mechanisms. This commit separates the pool/scheduling algorithm
tests into their own suite, and also simplifies those tests.

The pull request replaces #339.

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #3967 from kayousterhout/SPARK-1143 and squashes the following commits:

8a898c4 [Kay Ousterhout] [SPARK-1143] Separate pool tests into their own suite.