Unit tests

We need several unit tests to kickstart the project and avoid regressions:

• import & convert to pytest test_reporting from the worker-ml project (without the ml tool validation)
• import & convert to pytest test_image from the worker-ml project (with related fixtures)
• import & convert to pytest test_element from the worker-ml project
• import & convert to pytest test_elementlist from the worker-ml project (the ElementsList has been merged into ElementsWorker): it should be in test_elements_worker
• test_base is now useless, do nothing on this one
• create new tests in test_base_worker to validate:
  • api client creation
  • base cli args
• add new tests in test_elements_worker to validate:
  • elements related cli args
  • elements listing
  • create_sub_element