selftests/landlock: Add test to check partial access in a mount tree

Add layout1.refer_part_mount_tree_is_allowed to test the masked logical
issue regarding collect_domain_accesses() calls followed by the
is_access_to_paths_allowed() check in current_check_refer_path().  See
previous commit.

This test should work without the previous fix as well, but it enables
us to make sure future changes will not have impact regarding this
behavior.

Cc: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20250108154338.1129069-13-mic@digikod.net


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