selftests: mptcp: join: ability to invert ADD_ADDR check

In the following commit, the client will initiate the ADD_ADDR, instead
of the server. We need to way to verify the ADD_ADDR have been correctly
sent.

Note: the default expected counters for when the port number is given
are never changed by the caller, no need to accept them as parameter
then.

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: 86e39e04

 ("mptcp: keep track of local endpoint still available for each msk")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240731-upstream-net-20240731-mptcp-endp-subflow-signal-v1-6-c8a9b036493b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>