Adding runtime testing

[erikarvstedt]

While hacking on the activation script I noticed the lack of an infrastructure for
testing runtime features like generation activation and services.

Here's how runtime testing could be implemented:

• Run tests in a VM using the NixOS testing framework, which is very pleasant to use
  with the new Python backend.
• Add an interactive testing mode to avoid VM restarts while creating and debugging tests.
  • Besides interactively running tests, it should be possible to rebuild and activate
    changed test home configs from within testing Python REPL.
• To keep runtime overhead low, we can simply run multiple tests sequentially in a
  single VM. Between tests the home directory and user units are reset.
• nmt integration is simple: just add a test case with an empty script and a home
  config that references the test derivation.
• Travis GCE nodes support KVM, so the tests can be run by Travis.

Do you like the concept? I'll then flesh it out and add a PR.

[erikarvstedt]

This should be done in a container instead of a VM. Let's revisit this issue when unprivileged containers are implemented in nixpkgs.