Fix jacoco plugin

[koppor]

We used a deprecated gradle setting ($buidDir). Morevoer, JaCoCo did not cover the "normal" test cases. Both is fixed with this PR.

After executing ./gradlew jacocoTestReport, build/reports/jacoco/test/html/index.html is generated:

This PR also includes jacoco in testing.md (and restructures it a bit).

Mandatory checks

• [ ] Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• [ ] Tests created for changes (if applicable)
• [ ] Manually tested changed features in running JabRef (always required)
• [ ] Screenshots added in PR description (for UI changes)
• [ ] Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• [ ] Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[github-actions[bot]]

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