selftests/landlock: Factor out audit fixture in audit_test

The audit fixture needlessly stores and manages domain_stack.  Move it
to the audit.layers tests.  This will be useful to reuse the audit
fixture with the next patch.

Cc: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20250410171725.1265860-2-mic@digikod.net


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