selftests: mptcp: add mptcp_lib_wait_local_port_listen

selftests: mptcp: add mptcp_lib_wait_local_port_listen

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

wait_local_port_listen() helper is defined in diag.sh, mptcp_connect.sh,
mptcp_join.sh and simult_flows.sh, export it into mptcp_lib.sh and
rename it with mptcp_lib_ prefix. Use this new helper in all these
scripts.

Note: We only have IPv4 connections in this helper, not looking at IPv6
(tcp6) but that's OK because we only have IPv4 connections here in diag.sh.

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-v4-15-8d6b94150f6b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>