selftests: mptcp: increase timeout to 30 min

On very slow environments -- e.g. when QEmu is used without KVM --,
mptcp_join.sh selftest can take a bit more than 20 minutes. Bump the
default timeout by 50% as it seems normal to take that long on some
environments.

When a debug kernel config is used, this selftest will take even longer,
but that's certainly not a common test env to consider for the timeout.

The Fixes tag that has been picked here is there simply to help having
this patch backported to older stable versions. It is difficult to point
to the exact commit that made some env reaching the timeout from time to
time.

Fixes: d17b968b ("selftests: mptcp: increase timeout to 20 minutes")
Cc: stable@vger.kernel.org
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240131-upstream-net-20240131-mptcp-ci-issues-v1-5-4c1c11e571ff@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>