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

When OOB data is in recvq, we can detect it with epoll by checking
EPOLLPRI.

This patch add checks for EPOLLPRI after every send() and recv() in
all test cases.

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