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/tests/predictable-interface-names.nix

@@ -1,7 +1,10 @@
-{ system ? builtins.currentSystem }:
+{ system ? builtins.currentSystem,
+  config ? {},
+  pkgs ? import ../.. { inherit system config; }
+}:
 
 let
-  inherit (import ../lib/testing.nix { inherit system; }) makeTest pkgs;
+  inherit (import ../lib/testing.nix { inherit system pkgs; }) makeTest;
 in pkgs.lib.listToAttrs (pkgs.lib.crossLists (predictable: withNetworkd: {
   name = pkgs.lib.optionalString (!predictable) "un" + "predictable"
        + pkgs.lib.optionalString withNetworkd "Networkd";