- compiler warning
- detection of pthread_create failure (you will see this message
only if you run with "make test-verbose" in unittest; otherwise
unit.pl masks all messages from the test but "ok" ones.
- the test fails randomly on some machines (I filed it as BUG#22320),
on one host it looks like a crash at exit() which a sleep(2) makes
disappear. So I add the sleep(2), which can be removed
when BUG#22320 is fixed. 


unittest/mysys/my_atomic-t.c:
  - fix for compiler warning on 64-bit "cast from pointer to integer of
  different size". Casting to long and then to int. We'll use intptr
  instead later.
  - detect if pthread_create fails.
  - sleep(2) until BUG#22320 is fixed