robustify (deadlock-detection interrupts) test

robustify (deadlock-detection interrupts) test

    1. Make sure the desired "tricky" ordering happens.
    2. Make sure threads can't exit before they're intrerupted.

  Next time I commit a test that relies on SLEEP without a comment explaining
  (1) why it is necessary and there isn't a better alternative (2) what the
  ordering the tests is trying to achieve actually ise... please hit me with a
  stick. Hard.