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 
```python
def make_backup():
    ...

def restore_backup():
    ...

@pytest.fixture(scope='function')
def clean_system(request):
    make_backup()
    request.addfinalizer(restore_backup)
```
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