selftests/landlock: Extend tests for minimal valid attribute size

This might be useful when the struct landlock_ruleset_attr will get more
fields.

Cc: Shuah Khan <shuah@kernel.org>
Link: https://lore.kernel.org/r/20220506160820.524344-4-mic@digikod.net


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