Add a test to verify that the SIGURG signal created by an out-of-bound
message in UNIX sockets is well controlled by the file_send_sigiotask
hook.

Test coverage for security/landlock is 92.2% of 1046 lines according to
gcc/gcov-14.
Signed-off-by: Tahera Fahimi <fahimitahera@gmail.com>
Link: https://lore.kernel.org/r/50daeed4d4f60d71e9564d0f24004a373fc5f7d5.1725657728.git.fahimitahera@gmail.com
[mic: Improve commit message and add test coverage, improve test with
four variants to fully cover the hook, use abstract unix socket to avoid
managing a file, use dedicated variable per process, add comments, avoid
negative ASSERT, move close calls]
Co-developed-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Mickaël Salaün <mic@digikod.net>