test: explanation of MTIME_RESOLUTION resetting.

test: explanation of MTIME_RESOLUTION resetting.

* t/ax/test-defs.in (MTIME_RESOLUTION): separate the case
where am_cv_filesystem_timestamp_resolution is not set for
more explanations, but still reset MTIME_RESOLUTION to 1
even if it is already set. Which it should be when running
tests, because config.status is sourced. But it's too late in
the release process to change this now; after the release,
can try keeping a non-empty MTIME_RESOLUTION.