Clean up and externalise tests

This is mainly a clean-up, there are a couple of deps that are not needed any more (since using through2). I've also refactored the tests so that the test file exports a function that can set up the tests given a `pygmentize` function. This is because I've written a wrapper, [pygmentize-bundled-cached](https://github.com/rvagg/pygmentize-bundled-cached) with the same API except it caches results for faster serving. I want to test that it works exactly the same by reusing the tests.