Enable resetting purge and enhance unit tests

[soininen]

This salvages some additions that got lost when #333 was reverted.

No related issue.

Checklist before merging

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

[codecov-commenter]

Codecov Report

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

Comparison is base (30fc985) 77.32% compared to head (02130ed) 77.35%. Report is 10 commits behind head on 0.8-dev.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #349 +/- ## =========================================== + Coverage 77.32% 77.35% +0.02% =========================================== Files 75 75 Lines 9280 9283 +3 Branches 1958 1959 +1 =========================================== + Hits 7176 7181 +5 + Misses 1776 1775 -1 + Partials 328 327 -1 ```

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