selftests: mptcp: join: userspace: wait for new events

selftests: mptcp: join: userspace: wait for new events

Instead of waiting for a random amount of time (1 second), wait for an
event to be received on the other side.

To do that, when an address is announced (userspace_pm_add_addr), the
ANNOUNCED is expected. When a new subflow is created
(userspace_pm_add_sf), the SUB_ESTABLISHED event is expected.

With this, the tests can finish quicker.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260203-net-next-mptcp-misc-feat-6-20-v1-11-31ec8bfc56d1@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>