Split unit tests from test_elements_worker.py in sub files

That file has become unreadable (too many tests)

Just move the unit tests by topic:

• cli
• elements (list_elements + corpus + create sub element + list_element_children)
• classification (get_ml_class + create_classification
• transcriptions (create_transcription + create_element_transcription + list_transcriptions)
• entity (create_entity)
• metadata (create_metadata)
• worker (get_worker_version + ml_result + update activity)