Add test coverage for significant scap clean functions

utils.py:
get_active_wikiversions():
Update docstring to accurately describe how the representative
wikidb is selected.

Move the return_type sanity check to the front of the function so that
the codepath can be easily tested.

tests/scap/plugins/test_clean.py:
New tests.

tests/scap/test_tasks.py:
Test tasks.get_wikiversions_ondisk().

tests/scap/test_utils.py:
Test utils.get_active_wikiversions().

---

Related MRs:

• !309 (merged) (This MR)
• !308 (merged)
• !305 (merged)
• master

Depends-On: !308 (merged)