tests: refactor to carry the package set as an argument

This way, the package set will be possible to pass without re-importing
all the time

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

@@ -1,9 +1,13 @@
 { system ? builtins.currentSystem
+, config ? {}
 , networkExpr
 }:
 
 let nodes = import networkExpr; in
 
-with import ../../../../lib/testing.nix { inherit system; };
+with import ../../../../lib/testing.nix {
+  inherit system;
+  pkgs = import ../.. { inherit system config; }
+};
 
 (makeTest { inherit nodes; testScript = ""; }).driver