Fail parallel tests if workers exit early

Previously, if a test worker exited early, the in-flight test it was
supposed to run wasn't reported as a failure.

If all workers exited immediately, this would be reported as ex.

    Finished in 1.708349s, 39.2192 runs/s, 79.0237 assertions/s.
    67 runs, 135 assertions, 0 failures, 0 errors, 2 skips

This commit validates that all workers finish running tests by ensuring
that the queue is empty after they exit. This works because we signal
the workers to exit by pushing nil onto the queue, so that there should
be a number of items left in the queue matching potentially missed
tests.

0 files changed, 0 insertions, 0 deletions