selftests/landlock: Add tests for O_PATH

The O_PATH flag is currently not handled by Landlock.  Let's make sure
this behavior will remain consistent with the same ruleset over time.

Cc: Shuah Khan <shuah@kernel.org>
Link: https://lore.kernel.org/r/20220506160820.524344-8-mic@digikod.net


Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>