nixosTest: adds support for lib.extend

When lib overrides were used, before this commit, they would not be made
available in the configuration evaluation of nixosTest's nodes.

Sample code:
``` nix
let
  pkgs = import ./. {
    overlays = [
      (new: old: {
        lib = old.lib.extend (self: super: {
          sorry_dave = builtins.trace "There are no pod bay doors" "sorry dave";
        });
      })
    ];
  };
in
pkgs.testers.nixosTest {
  name = "demo lib overlay";

  nodes = {
    machine = { lib, ... }: {
      environment.etc."got-lib-overlay".text = lib.sorry_dave;
    };
  };

  testScript = { nodes }:
    ''
      start_all()
      machine.succeed('grep dave /etc/got-lib-overlay')
    '';
}
```

nixos/lib/testing/nodes.nix

@@ -16,6 +16,7 @@ let
 
   baseOS =
     import ../eval-config.nix {
+      inherit lib;
       system = null; # use modularly defined system
       inherit (config.node) specialArgs;
       modules = [ config.defaults ];