selftests/landlock: Test signal scoping

Provide tests for the signal scoping.  If the signal is 0, no signal
will be sent, but the permission of a process to send a signal will be
checked.  Likewise, this test consider one signal for each signal
category: SIGTRAP, SIGURG, SIGHUP, and SIGTSTP.

Signed-off-by: Tahera Fahimi <fahimitahera@gmail.com>
Link: https://lore.kernel.org/r/15dc202bb7f0a462ddeaa0c1cd630d2a7c6fa5c5.1725657728.git.fahimitahera@gmail.com


[mic: Fix commit message, use dedicated variables per process, properly
close FDs, extend send_sig_to_parent to make sure scoping works as
expected]
Signed-off-by: Mickaël Salaün <mic@digikod.net>