[issue20910] Make sleep configurable in tests

[STINNER Victor]

STINNER Victor added the comment:

"I've sped up "wait for something to happen" tests in other contexts and
found the best solution to be a polling loop so that success is as fast as possible and failure may be slow."

more_reliable_tests.patch tries to fix tests to not depend on timings. But it is not always possible to synchronize without "arbitrary sleep". 

test_sleep.patch helps to detect such unstable test.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20910>
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