selftest: af_unix: Add non-TCP-compliant test cases in msg_oob.c.

While testing, I found some weird behaviour on the TCP side as well.

For example, TCP drops the preceding OOB data when queueing a new
OOB data if the old OOB data is at the head of recvq.

  #  RUN           msg_oob.no_peek.ex_oob_drop ...
  # msg_oob.c:146:ex_oob_drop:AF_UNIX :x
  # msg_oob.c:147:ex_oob_drop:TCP     :Resource temporarily unavailable
  # msg_oob.c:146:ex_oob_drop:AF_UNIX :y
  # msg_oob.c:147:ex_oob_drop:TCP     :Invalid argument
  #            OK  msg_oob.no_peek.ex_oob_drop
  ok 9 msg_oob.no_peek.ex_oob_drop

  #  RUN           msg_oob.no_peek.ex_oob_drop_2 ...
  # msg_oob.c:146:ex_oob_drop_2:AF_UNIX :x
  # msg_oob.c:147:ex_oob_drop_2:TCP     :Resource temporarily unavailable
  #            OK  msg_oob.no_peek.ex_oob_drop_2
  ok 10 msg_oob.no_peek.ex_oob_drop_2

This patch allows AF_UNIX's MSG_OOB implementation to produce different
results from TCP when operations are guarded with tcp_incompliant{}.

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