selftest: af_unix: Add tests for -ECONNRESET.

selftest: af_unix: Add tests for -ECONNRESET.

A new function resetpair() calls close() for the receiver and checks
the return value from recv() on the initial sender side.

Now resetpair() is added to each test case and some additional test
cases.

Note that TCP sets -ECONNRESET to the consumed OOB, but we have decided
not to touch TCP MSG_OOB code in the past.

Before:

  #  RUN           msg_oob.no_peek.ex_oob_ex_oob ...
  # msg_oob.c:236:ex_oob_ex_oob:AF_UNIX :Connection reset by peer
  # msg_oob.c:237:ex_oob_ex_oob:Expected:
  # msg_oob.c:239:ex_oob_ex_oob:Expected ret[0] (-1) == expected_len (0)
  # ex_oob_ex_oob: Test terminated by assertion
  #          FAIL  msg_oob.no_peek.ex_oob_ex_oob
  not ok 14 msg_oob.no_peek.ex_oob_ex_oob
  ...
  # FAILED: 36 / 48 tests passed.
  # Totals: pass:36 fail:12 xfail:0 xpass:0 skip:0 error:0

After:

  #  RUN           msg_oob.no_peek.ex_oob_ex_oob ...
  # msg_oob.c:244:ex_oob_ex_oob:AF_UNIX :
  # msg_oob.c:245:ex_oob_ex_oob:TCP     :Connection reset by peer
  #            OK  msg_oob.no_peek.ex_oob_ex_oob
  ok 14 msg_oob.no_peek.ex_oob_ex_oob
  ...
  # PASSED: 48 / 48 tests passed.
  # Totals: pass:48 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250619041457.1132791-5-kuni1840@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>