overlay: run unionmount tests with custom overlay mount options

overlay: run unionmount tests with custom overlay mount options

Assign $OVERLAY_MOUNT_OPTIONS to UNIONMOUNT_MNTOPTIONS and require
that unionmount supports UNIONMOUNT_MNTOPTIONS if OVERLAY_MOUNT_OPTIONS
was provided.

For example, when the mount option metacopy=on is set in
$OVERLAY_MOUNT_OPTIONS, it enables the --meta test option and affects
the test verifications after copy up.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>