Add a unit test

[soininen]

Added a unit test to investigate a bug in Toolbox. Might as well commit it to the repository.

Re spine-tools/Spine-Toolbox#2417

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (3adfbce) 76.95% compared to head (55adad4) 76.95%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #311 +/- ## ======================================== Coverage 76.95% 76.95% ======================================== Files 74 74 Lines 9175 9175 Branches 1948 1948 ======================================== Hits 7061 7061 Misses 1780 1780 Partials 334 334 ```

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