Migrate to JUnit 5 and Mockito 4 + Java 8 Lambdas

[fabianfrz]

Refactor the test suite to be more state of the art.

• I migrated it to Mockito 4 since Easymock struggles on JDK 17.
• Migrate the project to JUnit 5 (not entirely beatifully but more like everything needed to get it running and change as less code as possible - junit 5 actually has some more beautiful tools as well like extensions etc.)
• Migrate anonymous inner classes to lambdas.