We regressed quiet into requiring a value to be set as a cli option (`--quiet=true`) instead of just functioning as a flag (`--quiet`). This change restores the previous interface on the command line, and adds a test to ensure no regressions.
Extract the individual parsing tests (cli, env, rc) and add additional configuration test file (TConf and config builder). To extract the fixtures they have to go into an additional 'conftest.py' file for pytest to recognize and automatically import them, see: https://docs.pytest.org/en/stable/reference/fixtures.html#conftest-py-sharing-fixtures-across-multiple-files and: https://docs.pytest.org/en/stable/how-to/fixtures.html#using-fixtures-from-other-projects