[issue16968] Fix test discovery for test_concurrent_futures.py

[Zachary Ware]

Zachary Ware <zachary.ware at gmail.com> added the comment:

Looking at this again after 5 years, things have changed a bit and I also better understand what's actually going on in the tests.  IIUC, the `test_main` function's only real purpose was to cause any dangling threads or processes to be reaped, which should actually be done by the shutdown methods of the tested executors.  Doing that cleanup in test_main (if successful), would actually prevent us from getting useful warnings from regrtest if shutdown did not clean up properly, so the attached PR simply removes test_main.

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versions: +Python 3.7, Python 3.8 -Python 3.4, Python 3.5

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