test: refactor first UDP test to use `UdpServer`
test: refactor first UDP test to use `UdpServer`

This makes the tests more uniform.

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

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit