Refactor `PluginTest` to eliminate duplication

[Vlatombe]

Eliminate all code duplication, using gson to format json instead of building strings manually.

Remove deprecated code.

Existing tests all pass.

Coverage remains the same, with the test class size divided by 2 (858 lines vs. 1520)

Testing done

Submitter checklist

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [ ] Link to relevant issues in GitHub or Jira
• [ ] Link to relevant pull requests, esp. upstream and downstream changes
• [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[codecov[bot]]

Codecov Report

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

Project coverage is 72.46%. Comparing base (bca3705) to head (764be9c). Report is 4 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #345 +/- ## ========================================= Coverage 72.46% 72.46% + Complexity 201 200 -1 ========================================= Files 9 9 Lines 839 839 Branches 119 119 ========================================= Hits 608 608 Misses 170 170 Partials 61 61 ```

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[Vlatombe]

Hi @michael-doubez, do you have time to review this ? Ideally I'd like to get it merged before I can add testing to #344, I don't want to add more duplications.