[3.6] bpo-30649: test_os tolerates 50 ms delta for utime (#2156) (#2175)

[3.6] bpo-30649: test_os tolerates 50 ms delta for utime (#2156) (#2175)

* bpo-30649: test_os tolerates 50 ms delta for utime (#2156)

On Windows, tolerate a delta of 50 ms instead of 20 ms in
test_utime_current() and test_utime_current_old() of test_os.

On other platforms, reduce the delta from 20 ms to 10 ms.
(cherry picked from commit c94caca65cd38802243b5279cf85ee44ffb2abb8)

* bpo-30649: Revert utime delta in test_os (#2176)

PPC64 Fedora 3.x buildbot requires at least a delta of 14 ms: revert
the utime delta to 20 ms.

I tried 10 ms, but test_os failed on the PPC64 Fedora 3.x buildbot.
(cherry picked from commit 3402f7268897db15053866e1e68404cfa0e02706)