selftest: af_unix: Add test for SO_PEEK_OFF.

selftest: af_unix: Add test for SO_PEEK_OFF.

The test covers various cases to verify SO_PEEK_OFF behaviour
for all AF_UNIX socket types.

two_chunks_blocking and two_chunks_overlap_blocking reproduce
the issue mentioned in the previous patch.

Without the patch, the two tests fail:

  #  RUN           so_peek_off.stream.two_chunks_blocking ...
  # so_peek_off.c:121:two_chunks_blocking:Expected 'bbbb' == 'aaaabbbb'.
  # two_chunks_blocking: Test terminated by assertion
  #          FAIL  so_peek_off.stream.two_chunks_blocking
  not ok 3 so_peek_off.stream.two_chunks_blocking

  #  RUN           so_peek_off.stream.two_chunks_overlap_blocking ...
  # so_peek_off.c:159:two_chunks_overlap_blocking:Expected 'bbbb' == 'aaaabbbb'.
  # two_chunks_overlap_blocking: Test terminated by assertion
  #          FAIL  so_peek_off.stream.two_chunks_overlap_blocking
  not ok 5 so_peek_off.stream.two_chunks_overlap_blocking

With the patch, all tests pass:

  # PASSED: 15 / 15 tests passed.
  # Totals: pass:15 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251117174740.3684604-3-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>