test: add a UDP `wait_for_traffic` function
test: add a UDP `wait_for_traffic` function

When combined with the `resend_all_replies`, this allows us to ask
a server to resend replies while we wait for new packets to appear.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit