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/doc/manual/release-notes/rl-2003.xml

@@ -625,6 +625,12 @@ auth required pam_succeed_if.so uid >= 1000 quiet
      to a fairly old snapshot  from the <package>gcc7</package>-branch.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     The <citerefentry><refentrytitle>nixos-build-vms</refentrytitle><manvolnum>8</manvolnum>
+     </citerefentry>-script now uses the python test-driver.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 

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; };
 };