selftest: af_unix: Check SIGURG after every send() in msg_oob.c

When data is sent with MSG_OOB, SIGURG is sent to a process if the
receiver socket has set its owner to the process by ioctl(FIOSETOWN)
or fcntl(F_SETOWN).

This patch adds SIGURG check after every send(MSG_OOB) call.

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