selftest: af_unix: Test SO_PASSRIGHTS.

scm_rights.c has various patterns of tests to exercise GC.

Let's add cases where SO_PASSRIGHTS is disabled.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>