facilitate deletion of coafile.green on running pytest

*Identification of tests is required which create `coafile.green` files
after which they must be deleted in those tests
* Even though https://github.com/coala/coala-quickstart/issues/309 adds them to the `.gitignore`, `coafile.green` should be created when actually running the `--green-mode` on the repository, not by pytest

Start by running pytest and identifying the tests that create coafile.green files. Inspect how --green-mode is handled, then ensure those test-created files are deleted while repository runs with --green-mode still create them; verify by rerunning the relevant pytest tests.

Written by the indexing model from the issue text.