selftests/landlock: Split signal_scoping_threads tests

selftests/landlock: Split signal_scoping_threads tests

Split signal_scoping_threads tests into signal_scoping_thread_before
and signal_scoping_thread_after.

Use local variables for thread synchronization.  Fix exported function.
Replace some asserts with expects.

Fixes: c8994965013e ("selftests/landlock: Test signal scoping for threads")
Cc: Günther Noack <gnoack@google.com>
Cc: Tahera Fahimi <fahimitahera@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250318161443.279194-7-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>