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

The helper verify_listener_events() is defined both in mptcp_join.sh and
userspace_pm.sh, export it into mptcp_lib.sh and rename it with mptcp_lib_
prefix. Use this new helper in both scripts.
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240308-upstream-net-next-20240308-selftests-mptcp-unification-v1-13-4f42c347b653@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>