selftests: mptcp: join: wait for estab event instead of MPJ

selftests: mptcp: join: wait for estab event instead of MPJ

'wait_mpj' was used just after having created a background connection,
but before creating new subflows. So no MPJ were sent. The intention was
to wait for the connection to be established, which was the same as
doing a simple sleep with a "random" value.

Instead, wait for an "established" event. 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-v1-9-31ec8bfc56d1@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>