selftest: af_unix: Add test case with mixed lowpoint in scm_rights.c.

The new test case creates two SCCs so that each of them
has multiple scc_index.

Without patch, GC cannot free the sockets and the test fails.

  #  RUN           scm_rights.dgram.mixed_lowpoints ...
  # scm_rights.c:176:mixed_lowpoints:Expected 0 (0) == ret (12)
  # mixed_lowpoints: Test terminated by assertion
  #          FAIL  scm_rights.dgram.mixed_lowpoints
  not ok 5 scm_rights.dgram.mixed_lowpoints
  ...
  # FAILED: 45 / 50 tests passed.
  # Totals: pass:45 fail:5 xfail:0 xpass:0 skip:0 error:0

With the patch, all tests pass.

  # PASSED: 50 / 50 tests passed.
  # Totals: pass:50 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260912030852.1467872-3-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>