selftests: mptcp: add a test for mptcp_diag_dump_one

This patch introduces a new 'chk_diag' test in diag.sh. It retrieves
the token for a specified MPTCP socket (msk) using the 'ss' command and
then accesses the 'mptcp_diag_dump_one' in kernel via ./mptcp_diag
to verify if the correct token is returned.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/524


Signed-off-by: Gang Yan <yangang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250228-net-next-mptcp-coverage-small-opti-v1-2-f933c4275676@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>