nixos/testing: Add pkgs parameter

This parameter is for packages to use in VMs, unlike hostPkgs.

nixos/lib/testing/pkgs.nix

@@ -0,0 +1,11 @@
+{ config, lib, hostPkgs, ... }:
+{
+  config = {
+    # default pkgs for use in VMs
+    _module.args.pkgs = hostPkgs;
+
+    defaults = {
+      # TODO: a module to set a shared pkgs, if options.nixpkgs.* is untouched by user (highestPrio) */
+    };
+  };
+}