nixos/tests/login: fix the seat test by loading sound drivers

It turned out that /dev/snd/* always exists even if there are no sound
drivers loaded at all. Loading `snd` and `snd_timer` fixes that
situation. It is probably fair to assume someone that wants to use sound
also enables that in the NixOS configuration.

nixos/tests/login.nix

@@ -9,6 +9,7 @@ import ./make-test.nix ({ pkgs, latestKernel ? false, ... }:
   machine =
     { pkgs, lib, ... }:
     { boot.kernelPackages = lib.mkIf latestKernel pkgs.linuxPackages_latest;
+      sound.enable = true; # needed for the factl test, /dev/snd/* exists without them but udev doesn't care then
     };
 
   testScript =