selftest: af_unix: Add msg_oob.c.

AF_UNIX's MSG_OOB functionality lacked thorough testing, and we found
some bizarre behaviour.

The new selftest validates every MSG_OOB operation against TCP as a
reference implementation.

This patch adds only a few tests with basic send() and recv() that
do not fail.

The following patches will add more test cases for SO_OOBINLINE, SIGURG,
EPOLLPRI, and SIOCATMARK.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>