Migrate tests to JUnit5

[strangelookingnerd]

This PR aims to migrate all tests to JUnit5. Changes include:

• Migrate annotations and imports
• Migrate assertions
• Remove public visibility for test classes and methods
• Minor clean up

I am well aware that this is a quite large changeset however I hope that there is still interest in this PR and it will be reviewed.
If there are any questions, please do not hesitate to ping me.

• Make sure you are requesting to pull a topic/feature/bugfix branch (right side) and not your master branch!
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• Please describe what you did
• Link to relevant issues in GitHub
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[asimell]

@strangelookingnerd Could you rebase this and resolve merge conflicts, please? I merged #83 this morning, which removed and refactored some tests.

[strangelookingnerd]

Rebased the PR. Bad timing on my end there 😄

[asimell]

No worries, thanks for quick reaction!