selftests: mptcp: join: validate fullmesh endp on 1st sf

commit 4878f9f8421f4587bee7b232c1c8a9d3a7d4d782 upstream.

This case was not covered, and the wrong ID was set before the previous
commit.

The rest is not modified, it is just that it will increase the code
coverage.

The right address ID can be verified by looking at the packet traces. We
could automate that using Netfilter with some cBPF code for example, but
that's always a bit cryptic. Packetdrill seems better fitted for that.

Fixes: 4f49d633

 ("selftests: mptcp: add fullmesh testcases")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20240819-net-mptcp-pm-reusing-id-v1-13-38035d40de5b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Conflicts in mptcp_join.sh, because the 'run_tests' helper has been
  modified in multiple commits that are not in this version, e.g. commit
  e571fb09

 ("selftests: mptcp: add speed env var"). The conflict was
  in the context, the new line can still be added at the same place. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>