selftests: net: move wait_local_port_listen to lib.sh

selftests: net: move wait_local_port_listen to lib.sh

The function wait_local_port_listen() is the only function defined in
net_helper.sh. Since some tests source both lib.sh and net_helper.sh,
we can simplify the setup by moving wait_local_port_listen() to lib.sh.

With this change, net_helper.sh becomes redundant and can be removed.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250526014600.9128-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>