selftests: mptcp: join: support local endpoint being tracked or not

Selftests are supposed to run on any kernels, including the old ones not
supporting all MPTCP features.

At some points, a new feature caused internal behaviour changes we are
verifying in the selftests, see the Fixes tag below. It was not a uAPI
change but because in these selftests, we check some internal
behaviours, it is normal we have to adapt them from time to time after
having added some features.

It is possible to look for "mptcp_pm_subflow_check_next" in kallsyms
because it was needed to introduce the mentioned feature. So we can know
in advance what the behaviour we are expecting here instead of
supporting the two behaviours.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368


Fixes: 86e39e04 ("mptcp: keep track of local endpoint still available for each msk")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>