selftests/landlock: Add PID tests for audit records

Add audit.thread tests to check that the PID tied to a domain is not a
thread ID but the thread group ID.  These new tests would not pass
without the previous TGID fix.

Extend matches_log_domain_allocated() to check against the PID that
created the domain.

Test coverage for security/landlock is 93.6% of 1524 lines according to
gcc/gcov-14.

Cc: Christian Brauner <brauner@kernel.org>
Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20250410171725.1265860-3-mic@digikod.net


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