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

nixos/tests/ferm.nix

@@ -23,6 +23,7 @@ import ./make-test.nix ({ pkgs, ...} : {
           networking = {
             dhcpcd.enable = false;
             useNetworkd = true;
+            useDHCP = false;
             interfaces.eth1.ipv6.addresses = mkOverride 0 [ { address = "fd00::1"; prefixLength = 64; } ];
             interfaces.eth1.ipv4.addresses = mkOverride 0 [ { address = "192.168.1.1"; prefixLength = 24; } ];
           };

nixos/tests/networking.nix

@@ -450,6 +450,7 @@ let
       name = "Virtual";
       machine = {
         networking.useNetworkd = networkd;
+        networking.useDHCP = false;
         networking.interfaces.tap0 = {
           ipv4.addresses = [ { address = "192.168.1.1"; prefixLength = 24; } ];
           ipv6.addresses = [ { address = "2001:1470:fffd:2096::"; prefixLength = 64; } ];