nixos/tests/{ferm,networking}: fix eval with networkd

The networking.virtual test does not work with networkd yet, for
multiple reasons:

- network-online.target is not reached, because tun0 and tap0 are
  considered as required for online but _not_ brought up or assigned
  the configured addresses
- the commands later in the test rely on some units from the scripted
  network setup

cc @fpletz networkd exper
cc @globin we looked at this together