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.)

nixos/tests/misc.nix

@@ -1,8 +1,6 @@
 # Miscellaneous small tests that don't warrant their own VM run.
 
-{ pkgs, ... }:
-
-{
+import ./make-test.nix {
 
   machine =
     { config, pkgs, ... }: