test: force long sleep with explicit cache variable setting.

test: force long sleep with explicit cache variable setting.

From https://bugs.gnu.org/67670.

* t/ax/test-defs.in: when MTIME_RESOLUTION is set to 1
because automake/autom4te do not support subsecond-mtime,
also set am_cv_sleep_fractional_seconds=false. Otherwise
the random parallel test failures from given bug and others return.
* m4/sanity.m4 (_AM_SLEEP_FRACTIONAL_SECONDS): reformat.
(_AM_FILESYSTEM_TIMESTAMP_RESOLUTION): elide "the" in AC_CACHE_CHECK.
* t/amassign.sh: end with :.
* HACKING: more on debugging tests.
* NEWS: update.