selftests: mptcp: add mptcp_lib_kill_wait

selftests: mptcp: add mptcp_lib_kill_wait

To avoid duplicated code in different MPTCP selftests, we can add
and use helpers defined in mptcp_lib.sh.

Export kill_wait() helper in userspace_pm.sh into mptcp_lib.sh and
rename it as mptcp_lib_kill_wait(). It can be used to instead of
kill_wait() in mptcp_join.sh. Use the new helper in both scripts.

Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231128-send-net-next-2023107-v4-9-8d6b94150f6b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>