nixos/tests/simple.nix: Include minimal.nix

This reduces the amount of rebuilds (particularly some X11 stuff)
after changing some dependencies like systemd.

nixos/tests/simple.nix

···
     maintainers = [ eelco ];
   };
 
-  machine = { config, pkgs, ... }: { };
+  machine = { config, pkgs, ... }: {
+    imports = [ ../modules/profiles/minimal.nix ];
+  };
 
   testScript =
     ''