test: add TCP loopback connection test
test: add TCP loopback connection test

This test runs a server on localhost, connects to it via vpnkit,
writes 1 KiB, 1 MiB and 1 GiB of data through the connection and
then reads the response.

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

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit