selftests: mptcp: add inq test case

client & server use a unix socket connection to communicate
outside of the mptcp connection.

This allows the consumer to know in advance how many bytes have been
(or will be) sent by the peer.
This allows stricter checks on the bytecounts reported by TCP_INQ cmsg.

Suggested-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>