Improve cleanup of processes that don't terminate.

btest-bg-wait signaled btest-bg-run-helper processes to terminate in
the event of a timeout, but that did not always follow through with
making sure to kill off the actual process on which it was waiting.
Merge remote-tracking branch 'origin/topic/jsiwek/misc'

* origin/topic/jsiwek/misc:
  Improve cleanup of processes that don't terminate.