Replace wireguard-go with GotaTun in test-manager

Unfortunately, creating a TUN device requires the test-manager to run as root, since macOS has
nothing like setcap in Linux. I implemented this by the `test-manager` sudoing itself if it's
not invoked as root. We did already require passwordless sudo on the test server for the same
reason (creating the tun device using wireguard-go), so this _should_ be fine.

Author: hulthe