selftest: af_unix: Check SIOCATMARK after every send()/recv() in msg_oob.c.

To catch regression, let's check ioctl(SIOCATMARK) after every
send() and recv() calls.

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