utimens-tests: port to NFS file systems

utimens-tests: port to NFS file systems

Testing on Solaris 8 with NFS: creat() and utimens(,NULL) seem to
set timestamps according to the current time on the server, while
utimens(,{,UTIME_NOW}) sets timestamps according to the current
time on the client.  If two machines are not perfectly
synchronized in time, then this makes time appear to move
backwards.  Avoid spurious test failures caused by a mtime
comparison across machines, by instead doing 2 mtime comparisons,
each known to be from timestamps tied to a single machine.

* tests/test-utimens.h (test_utimens): Add a utimens call prior to
grabbing stat buffer.

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