nixos/nixos-build-vms: switch to python test-driver

In 0945178b3c we decided that Perl-based
VM tests should be deprecated and will be removed between 20.03 and
20.09. So let's switch `nixos-build-vms(8)` to python as well (which is
entirely interactive, so other scripts won't break).

In my experience, the test-driver isn't used most of the time, so this
patch is mainly supposed to get rid of the (probably misleading)
deprecation warning when running `nixos-build-vms`. Apart from that, the
interface for python's test-driver is way nicer.

nixos/modules/installer/tools/nixos-build-vms/build-vms.nix

@@ -5,7 +5,7 @@
 
 let nodes = import networkExpr; in
 
-with import ../../../../lib/testing.nix {
+with import ../../../../lib/testing-python.nix {
   inherit system;
   pkgs = import ../../../../.. { inherit system config; };
 };