Mock the hell out of `yo` status code test

This replaces calls to `process.exit` and `Generator.lookup` with fakes so we
can test that `yo` correctly fails when an non-existing generator name is
provided without waiting for 50+ seconds.

Before you click on the "Files" tab:

![Don't!](http://www.reactiongifs.com/wp-content/uploads/2012/09/censorship.gif)