Unit tests for AnswersMap & unit test directory

[RR5555]

This PR adds unit tests for AnswersMap from _user_data.py.

It creates a test/unittest directory for unit test files, and starts it with test__user_data.py.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.15%. Comparing base (3ae340d) to head (8981621).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2454      +/-   ##
==========================================
- Coverage   97.26%   97.15%   -0.12%     
==========================================
  Files          55       56       +1     
  Lines        6299     6329      +30     
==========================================
+ Hits         6127     6149      +22     
- Misses        172      180       +8     

Flag	Coverage Δ
unittests	97.15% <100.00%> (-0.12%)	⬇️

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[sisp]

Thanks for your initiative towards improving our testing, @RR5555! 👍

TBH, I'm a bit hesitant about adding many unit tests – especially for this very internal AnswersMap class – because (a) @yajo advocates full application tests in a Podcast (approx. 39 min into the discussion) about Copier, and (b) I'm not sure about the added value given the extensive test suite we have.

[yajo]

I agree with @sisp. Please could you explain why did you add this proposal?