[issue16968] Fix test discovery for test_concurrent_futures.py

[Brett Cannon]

Brett Cannon added the comment:

The way I would replace test_main() would be:

if __name__ == '__main__':
  try:
    test.support.reap_threads(unittest.main)()
  finally:
    test.support_reap_children()

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue16968>
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