selftests: mptcp: join: recreate signal endp with same ID

selftests: mptcp: join: recreate signal endp with same ID

In this "delete re-add signal" MPTCP Join subtest, the endpoint linked
to the initial subflow is removed, but readded once with different ID.

It appears that there was an issue when reusing the same ID, recently
fixed by commit d191101dee25 ("mptcp: pm: in-kernel: always set ID as
avail when rm endp"). The test then now reuses the same ID the first
time, but continue to use another one (88) the second time.

This should then cover more cases.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/615
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260403-net-next-mptcp-msg_eor-misc-v1-5-b0b33bea3fed@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>