makefs: tests: Fix timestamp-related tests

Use a variable for mtree that includes the DEFAULT_MTREE_KEYWORDS, in
order to avoid missing the important "time" keyword or passing an
unimplemented "nlink" keyword.

Unskip the "failing" tests, and comment out the specific failing checks,
these will be addressed (or at least discussed) in a different revision.

For MS-DOS tests, use an even value timestamp, as precision is 2s.

For ZFS tests, use import_image instead of mount_image consistently.

Reviewed by: bnovkov, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54429