selftests/landlock: Add test to check rule tied to covered mount point

This test checks that a rule on a directory used as a mount point does
not grant access to the mount covering it.  It is a generalization of
the bind mount case in layout3_fs.hostfs.release_inodes [1] that tests
hidden mount points.

Cc: Günther Noack <gnoack@google.com>
Cc: Song Liu <song@kernel.org>
Cc: Tingmao Wang <m@maowtm.org>
Link: https://lore.kernel.org/r/20250606.zo5aekae6Da6@digikod.net [1]
Link: https://lore.kernel.org/r/20250606110811.211297-1-mic@digikod.net


Signed-off-by: Mickaël Salaün <mic@digikod.net>