test-stat-time: port to mingw

test-stat-time: port to mingw

Newer mingw has usleep, but it rejects arguments over 1000000
without sleeping.  And since stat has no visibility into
sub-second resolutions, it meant all the timestamps ended up
identical.  Fixed by restoring the 8 seconds of sleep, as
well as working around the documented unlink issue.

* tests/test-stat-time.c (force_unlink): Return a value.
(test_ctime) [W32]: Fix compilation error.
(nap): Don't call usleep with too large an argument.  Use
force_unlink.
* doc/pastposix-functions/usleep.texi (usleep): Document the
portability issue.

Signed-off-by: Eric Blake <ebb9@byu.net>