Add unit testing

[douglascdev]

The package currently relies completely on manual testing. Start using mocked tests:

• Mock pygame calls.
• If needed/desired, refactor any parts of the code that are unnecessarily coupled to make testing easier.
• Test the functions in the utils.py module.
• Test the methods in the PyPixelArt class, except run_loop(I don't think it's all that necessary to test it since it pretty much only calls PyPixelArt methods to work).