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.