Improve config tests

[johannaengland]

Scope and purpose

This PR improves the config tests added in #249 by using already existing fixtures and little things from pytest that I learned about during EuroPython.

Contributor Checklist

Every pull request should have this checklist filled out, no matter how small it is. More information about contributing to Zino can be found in the README.

• [ ] Added a changelog fragment for towncrier
  • only changes to the tests, no externally visible changes
• [X] Added/amended tests for new/changed code
• [X] Added/changed documentation
• [X] Linted/formatted the code with black, ruff and isort, easiest by using pre-commit
• [X] The first line of the commit message continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See https://cbea.ms/git-commit/
• [ ] If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done

[codecov[bot]]

Codecov Report

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

Project coverage is 98.60%. Comparing base (85a95ea) to head (0c42894). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #325 +/- ## ======================================= Coverage 98.60% 98.60% ======================================= Files 77 77 Lines 9688 9688 ======================================= Hits 9552 9552 Misses 136 136 ```

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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud