selftests/landlock: Test connected and unconnected datagram UNIX socket

Check the specific case where a scoped datagram socket is connected and
send(2) works, whereas sendto(2) is denied if the datagram socket is not
connected.

Signed-off-by: Tahera Fahimi <fahimitahera@gmail.com>
Link: https://lore.kernel.org/r/c28c9cd8feef67dd25e115c401a2389a75f9983b.1725494372.git.fahimitahera@gmail.com


[mic: Use more EXPECT and avoid negative ASSERT, use variables dedicated
per process, remove useless buffer]
Signed-off-by: Mickaël Salaün <mic@digikod.net>