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.
You can now run a test in the nixos/tests directory directly using
nix-build, e.g.
$ nix-build '<nixos/tests/login.nix>' -A test
This gets rid of having to add the test to nixos/tests/default.nix.
(Of course, you still need to add it to nixos/release.nix if you want
Hydra to run the test.)
