[SPARK-19560] Improve DAGScheduler tests.

This commit improves the tests that check the case when a
ShuffleMapTask completes successfully on an executor that has
failed.  This commit improves the commenting around the existing
test for this, and adds some additional checks to make it more
clear what went wrong if the tests fail (the fact that these
tests are hard to understand came up in the context of markhamstra's
proposed fix for #16620).

This commit also removes a test that I realized tested exactly
the same functionality.

markhamstra, I verified that the new version of the test still fails (and
in a more helpful way) for your proposed change for #16620.

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #16892 from kayousterhout/SPARK-19560.