selftests/landlock: Don't create useless file layouts

Add and use a layout0 test fixture to not populate the tmpfs filesystem
if it is not required for tests: unknown_access_rights, proc_nsfs,
unpriv and max_layers.

This doesn't change these tests but it speeds up their setup and makes
them less prone to error.  This prepare the ground for a next commit.

Link: https://lore.kernel.org/r/20230612191430.339153-3-mic@digikod.net


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