ZTS: handle FreeBSD version numbers correctly (#16340)

FreeBSD patchlevel versions are optional and, if present, in a different
location in the version string.

Sponsored-by: https://despairlabs.com/sponsor/

Signed-off-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>