Add tests for Todo.computed_duration and end properties.

[rafalkrupinski]

Follow up to previous PR

end when there is a start date but not due/duration (effectively testing the default)
computed_duration when there is no start or duration

Also fixed test function name conflict I've introduced

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.15%. Comparing base (7ca4829) to head (ef581ad).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #551      +/-   ##
==========================================
+ Coverage   97.08%   97.15%   +0.06%     
==========================================
  Files          53       53              
  Lines        3229     3229              
==========================================
+ Hits         3135     3137       +2     
+ Misses         94       92       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[allenporter]

Thank you!