selftests/landlock: Test audit with restrict flags

selftests/landlock: Test audit with restrict flags

Add audit_exec tests to filter Landlock denials according to
cross-execution or muted subdomains.

Add a wait-pipe-sandbox.c test program to sandbox itself and send a
(denied) signals to its parent.

Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20250320190717.2287696-24-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>