ZTS: zfs_mount_all_fail leaves /var/tmp/testrootPIDNUM directory around

Before we can remove test files, we need to unmount datasets
used by test first.

See also: zfs_mount_all_mountpoints.ksh

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Toomas Soome <tsoome@me.com>
Closes #16914