selftests: mptcp: lib: use wait_local_port_listen helper

selftests: mptcp: lib: use wait_local_port_listen helper

This patch includes net_helper.sh into mptcp_lib.sh, uses the helper
wait_local_port_listen() defined in it to implement the similar mptcp
helper. This can drop some duplicate code.

It looks like this helper from net_helper.sh was originally coming from
MPTCP, but MPTCP selftests have not been updated to use it from this
shared place.

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/20240607-upstream-net-next-20240607-selftests-mptcp-net-lib-v1-6-e36986faac94@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>