selftests/landlock: Test signal scoping for threads

Expand the signal scoping tests with pthread_kill(3).  Test if a scoped
thread can send signal to a process in the same scoped domain, or a
non-sandboxed thread.

Signed-off-by: Tahera Fahimi <fahimitahera@gmail.com>
Link: https://lore.kernel.org/r/c15e9eafbb2da1210e46ba8db7b8907f5ea11009.1725657728.git.fahimitahera@gmail.com


[mic: Improve commit message]
Signed-off-by: Mickaël Salaün <mic@digikod.net>