Fix rocky katalogus tests and delete unused fixtures

[dekkers]

Changes

While looking at the coverage of the rocky tests I saw that some lines didn't have coverage:

• I have removed the unused fixtures
• I fixed test_valid_plugin_id, the old code only tested if one of the three lines would throw an exception
• A number of katalogus tests didn't test start with test_ so weren't executed. I've fixed the name and updated the tests, because they never executed they didn't work anymore.

QA notes

Only test code is changed, so no QA needed.

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Checklist for code reviewers:

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Checklist for QA:

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud