test-utimens: speed up by taking shorter naps

test-utimens: speed up by taking shorter naps

* tests/nap.h (lt_mtime, get_mtime, nap_works, guess_delay):
New functions.
(nap): Use them, to do a better job of guessing the delay.
On Fedora 17 with ext4 atop md atop hard disks, this made
test-utimens run 10x faster, because the test napped for
1 ms at a time rather than 20 ms.  Reported by Stefano Lattarini in
<http://bugs.gnu.org/12820#11>.