Slurm was complaining because threadring was calling exit(0).  Cause that not
to happen, and note that these tests will have it happen if they are run with
Slurm.
Quiet new string tests.

- Throw --dataParTasksPerLocale=1 for promotion.chpl.  Using one tasks
  leads to consistent results and still tests what was originally
  intended (promotion, not task creation).

- Clarify comment in promotion.chpl

- Add another leakable type to begin.prediff (string reference count).

- Flush writes before checking memory to make sure that I/O buffers
  are freed.
Merge pull request #474 from sungeunchoi/test

Quiet new string tests.

- Throw --dataParTasksPerLocale=1 for promotion.chpl.  Using one tasks
  leads to consistent results and still tests what was originally
  intended (promotion, not task creation).

- Clarify comment in promotion.chpl

- Add another leakable type to begin.prediff (string reference count).

- Flush writes before checking memory to make sure that I/O buffers
  are freed.
Additions I forgot in the last commit.
Merge pull request #475 from sungeunchoi/test

Additions I forgot in the last commit.

These should have been part of #474.  I blame my cold.
Merge pull request #472 from lydia-duncan/silenceThreadringSlurm

Fix threadring slurm complaint

Slurm was complaining because threadring was calling exit(0). Cause that not
to happen, and note that these tests will have it happen if they are run with
Slurm.