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 
The original test uses `logging`, as does the re-implementation. However `py.test` only displays stdout in case the according test fails. It's up to you.