Upgrade tests to run with JUnit5

[strangelookingnerd]

• removed JUnit4 dependency
• migrated namespaces

Testing done

mvn clean verify runs without problems.

### 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
- [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[MarkEWaite]

I labeled the issue as a test (test improvement task) so that it would be classified by release drafter under one of the common headings in the changelog. You're welcome to assign it a different label if you prefer something else