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.