ZTS: functional/mount scripts are not removing /var/tmp/testdir.X dirs
cleanup.ksh is assuming we have TESTDIRS set.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Toomas Soome <tsoome@me.com>
Closes #16915