selftests: mptcp: join: validate new laminar endp

selftests: mptcp: join: validate new laminar endp

Here are a few sub-tests for mptcp_join.sh, validating the new 'laminar'
endpoint type.

In a setup where subflows created using the routing rules would be
rejected by the listener, and where the latter announces one IP address,
some cases are verified:

- Without any 'laminar' endpoints: no new subflows are created.

- With one 'laminar' endpoint: a second subflow is created.

- With multiple 'laminar' endpoints: 2 IPv4 subflows are created.

- With one 'laminar' endpoint, but the server announcing a second IP
  address, only one subflow is created.

- With one 'laminar' + 'subflow' endpoint, the same endpoint is only
  used once.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250927-net-next-mptcp-rcv-path-imp-v1-8-5da266aa9c1a@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>