Address “autoconf: forbidden tokens, basic” testsuite failure

Address “autoconf: forbidden tokens, basic” testsuite failure

On file systems with coarse-grained timestamps, this test was broken by
<https://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=713d9822bbfb2923115065efaefed34a0113f8a1>,
which changed autom4te's logic for deciding whether its output file is
newer than its cache file.  After that commit, if their modification
times are equal, the output is considered out of date.  Since both
files are created in quick succession, on file systems with coarse
timestamps (1 or 2s resolution), it is very easy for their
modification times to be equal.

As a stopgap for 2.72, in this test, force the generated configure
script’s mtime to be newer than the cache file.

* tests/tools.at (autoconf: forbidden tokens, basic): After the
  second AT_MTIME_DELAY, touch configure.