meta(nix): Move test lib to separate directory

lib/test.nix

@@ -0,0 +1,16 @@
+test: { self, pkgs }:
+let
+  inherit (pkgs.lib) mkDefault;
+
+  nixos-lib = import (pkgs.path + "/nixos/lib") { };
+in
+(nixos-lib.runTest {
+  imports = [ test ];
+
+  hostPkgs = pkgs;
+  defaults.documentation.enable = mkDefault false;
+
+  node.specialArgs = {
+    nix-config = self;
+  };
+}).config.result