test_utime.patch: a much larger patch which rewrites all unit tests on os.utime().

Changes:

* Use a fixed timestamp instead of copying timestamps from an existing file. If the timestamp of the original file can have a resolution of 1 nanosecond, os.utime() rounds to a resolution of 1 us on some platforms (when the C function uses a structure with a resolutionf of 1 us).
* Use a fixed timestamp with a resolution of 1 us instead of a resolution of 1 ms.
* Remove test_1565150(): it's now redundant with test_utime() and many other test_utime_*() tests
* Use self.fname instead of __file__ to check if the filesystem supports subsecond resolution: these two files may be in two different filesystems
* test_large_time() now checks the filesystem when it is executed, not when the class is defined. This change is to ensure that we are testing the right filesystem.
* replace support.TESTFN with self.dirname for readability
* move all os.utime() tests in a new dedicated class
* _test_utime_current() now get the system clock using time.time() and tolerate a delta of 10 ms instead of 10 seconds: we may increase the delta because of slow buildbots, but I hope that we can find a value smaller than 10 seconds!
* Avoid tricky getattr(), it's no more needed
* Merge duplicated test_utime_*() and test_utime_subsecond_*() functions
* Test also st_atime on when testing os.utime() on a directory
* etc.