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 `finalizer` needs access to the variables used in the `setup code` of the fixture, which is why I am using a local function definition at this point. The original `unittest.TestCase` used class attributes to that end.

I will extend the documentation to illustrate the procedure.