selftest: af_unix: Add Kconfig file.

diag_uid selftest failed on NIPA where the received nlmsg_type is
NLMSG_ERROR [0] because CONFIG_UNIX_DIAG is not set [1] by default
and sock_diag_lock_handler() failed to load the module.

  # # Starting 2 tests from 2 test cases.
  # #  RUN           diag_uid.uid.1 ...
  # # diag_uid.c:159:1:Expected nlh->nlmsg_type (2) == SOCK_DIAG_BY_FAMILY (20)
  # # 1: Test terminated by assertion
  # #          FAIL  diag_uid.uid.1
  # not ok 1 diag_uid.uid.1

Let's add all AF_UNIX Kconfig to the config file under af_unix dir
so that NIPA consumes it.

Fixes: ac011361

 ("af_unix: Add test for sock_diag and UDIAG_SHOW_UID.")
Link: https://netdev-3.bots.linux.dev/vmksft-net/results/644841/104-diag-uid/stdout [0]
Link: https://netdev-3.bots.linux.dev/vmksft-net/results/644841/config [1]
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20240617073033.0cbb829d@kernel.org/
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>