ZTS: Fix verify_fs_mount in delegate_common.kshlib

verify_fs_mount expects the dataset to remain unmounted after
updating the mountpoint property in delegate_common.kshlib.

This commit updates verify_fs_mount and uses nomount parameter
for zfs set to update the mountpoint property without mounting
the dataset.

This fixes the zfs_allow_010_pos test case, which was failing on
FreeBSD after the behavior update in setting the mountpoint
property.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #15376