Make it easier to run the tests

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.)