tests/nap.h: use an adaptive delay to avoid ctime update issues

The recent change in nap.h (5191133e) decreased the probability of lost
races to about a third, however such problems could still be observed
in virtual machines and openSUSE's OBS.

Instead of calulating the nap() time once and using it (together with
a small correction multiplier), avoid the race alltogether by verifying
on a reference file whether a timestamp difference has happened.

Before, nap() detected the needed time once empirically and then used
that delay (together with a small correction multiplier) in further
calls.  This problem has been reported and discussed several times,
including guesses about possible kernel issues:
  https://lists.gnu.org/archive/html/bug-gnulib/2013-04/msg00071.html
  http://lists.gnu.org/archive/html/coreutils/2012-03/msg00088.html
  https://lists.gnu.org/archive/html/bug-gnulib/2011-11/msg00226.html
  http://bugs.gnu.org/12820
  https://lists.gnu.org/archive/html/bug-gnulib/2010-11/msg00113.html
  https://lists.gnu.org/archive/html/bug-gnulib/2009-11/msg00007.html

Now, nap() avoids the race alltogether by verifying on a reference
file whether a timestamp difference has happened.

* tests/nap.h (nap_fd): Define file descriptor variable for the
witness file.
(nap_works): Change return value to bool.  Change passing
the old file's status by value instead of by reference as this function
does no longer update that timestamp; rename the function argument from
st to old_st.  Remove the local variables cdiff and mdiff because that
function now returns true/false instead of the precise delay.
(guess_delay): Remove function.
(clear_tmp_file): Add new function to close and unlink the witness file.
(nap): Instead of re-using the delay which has been calculated during
the first call, avoid the race by actually verifying that a timestamp
difference can be observed on the current file system.  Use an adaptive
approach for the delay to minimize execution time.  Assert that the
maximum delay is <= ~2 seconds, more precisely sum(2^n) from 0 to 30
= 2^31 - 1 = 2.1s.
Use atexit to call clear_tmp_file when the process terminates.