af_unix: Add test for consecutive consumed OOB.

Let's add a test case where consecutive concumed OOB skbs stay
at the head of the queue.

Without the previous patch, ioctl(SIOCATMARK) assertion fails.

Before:

  #  RUN           msg_oob.no_peek.ex_oob_ex_oob_oob ...
  # msg_oob.c:305:ex_oob_ex_oob_oob:Expected answ[0] (0) == oob_head (1)
  # ex_oob_ex_oob_oob: Test terminated by assertion
  #          FAIL  msg_oob.no_peek.ex_oob_ex_oob_oob
  not ok 12 msg_oob.no_peek.ex_oob_ex_oob_oob

After:

  #  RUN           msg_oob.no_peek.ex_oob_ex_oob_oob ...
  #            OK  msg_oob.no_peek.ex_oob_ex_oob_oob
  ok 12 msg_oob.no_peek.ex_oob_ex_oob_oob

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250619041457.1132791-3-kuni1840@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>