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 
By convention `py.test`searches for `conftest.py` files in testing directories for global hooks. Hence I'd like to stick to the filename, please see http://pytest.org/latest/plugins.html#conftest-py-local-per-directory-plugins for detailed info