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/keymap.nix

@@ -1,6 +1,9 @@
-{ system ? builtins.currentSystem }:
+{ system ? builtins.currentSystem,
+  config ? {},
+  pkgs ? import ../.. { inherit system config; }
+}:
 
-with import ../lib/testing.nix { inherit system; };
+with import ../lib/testing.nix { inherit system pkgs; };
 
 let
   readyFile  = "/tmp/readerReady";