selftests: mptcp: join: test for prohibited MPC to port-based endp

Explicitly verify that MPC connection attempts towards a port-based
signal endpoint fail with a reset.

Note that this new test is a bit different from the other ones, not
using 'run_tests'. It is then needed to add the capture capability, and
the picking the right port which have been extracted into three new
helpers. The info about the capture can also be printed from a single
point, which simplifies the exit paths in do_transfer().

The 'Fixes' tag here below is the same as the one from the previous
commit: this patch here is not fixing anything wrong in the selftests,
but it validates the previous fix for an issue introduced by this commit
ID.

Fixes: 1729cf18 ("mptcp: create the listening socket for new port")
Cc: stable@vger.kernel.org
Co-developed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20241014-net-mptcp-mpc-port-endp-v2-2-7faea8e6b6ae@kernel.org


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