test-stat-time: port to buggy NFS clients

test-stat-time: port to buggy NFS clients

On darwin, the NFS client reports mtime with st_sec==INT_MAX and
st_nsec monotonically increasing per transaction until the next
sync(); but sync() is expensive, so it is easier to just skip
this part of the test if mtime is nowhere near ctime.

* tests/test-stat-time.c (main) [W32]: Reduce ifdefs.
(test_ctime): Also skip test if mtime and ctime are skewed.

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