Previously systemd-networkd.service ran as systemd-network:nogroup.
The wireguard private key file is now owned by root:systemd-network with
mode 0640. It is therefore required that the systemd-network user is in the group
with the same name, so that it is able to read the key file.
Updating `systemd-networkd-wireguard` to use the python test runner.
This change was purely syntactic. This migration did not require any
semantic change.
