Improve move_mount01 coverage

[metan-ucw]

• Does not test if repeated move_mount() moves the mount point to a new place.
• Does not cover MOVE_MOUNT_SET_GROUP and MOVE_MOUNT_BENEATH

[metan-ucw]

The MOVE_MOUNT_BENEATH causes the mount point to be mounted under the current mount, that means that the test would need to mount a mntpoint first then mount another mount with MOVE_MOUNT_BENEATH on the same mount point, check that the firstly mounted filesystem is there, then mount the mount point and check that the filesystem mounted with MOVE_MOUNT_BENEATH is mounted at mntpoint now.

[metan-ucw]

The MOVE_MOUNT_SET_GROUP sets the event propagation group, this is related to the statmount() testcases where you can get the mount group ID.