Migrate JUnit 4 tests to JUnit Jupiter tests

[mguenther]

Kafka for JUnit is being tested using JUnit 4. We'd like to get rid of that dependency and migrate every unit test to a JUnit Jupiter based test. The updated test base will also serve as an example for writing JUnit tests with kafka-junit that leverage JUnit Jupiter. Hence, the documentation needs to be adapted as well.

[mguenther]

Merged into development.