Hackebrot convert tests to pytest

Convert a copy of `CookiecutterCleanSystemTestCase` to `py.test` syntax and use it for a re-implementation of `TestGenerateFiles.test_generate_files_nontemplated_exception` as a proof of concept. #256 
I don't think that we really need it at this point. I put it in there by habit. I'll remove the shebang wherever it is not being used.