selftests: mptcp: join: test for flush/re-add endpoints

selftests: mptcp: join: test for flush/re-add endpoints

After having flushed endpoints that didn't cause the creation of new
subflows, it is important to check endpoints can be re-created, re-using
previously used IDs.

Before the previous commit, the client would not have been able to
re-create the subflow that was previously rejected.

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: 06faa2271034 ("mptcp: remove multi addresses and subflows in PM")
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-v1-6-38035d40de5b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>