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.)
Simplify running tests even further

Now you can just say:

  $ nix-build '<nixos/tests/login.nix>'

You can still get the driver script for interactive testing:

  $ nix-build '<nixos/tests/login.nix>' -A driver
  $ ./result/bin/nixos-test-driver
Add script to create and upload GCE image.
kde-4.10: Remove
Fix installer test evaluation
Rewrite ‘with pkgs.lib’ -> ‘with lib’

Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.