Mockito extension #124
Description
Imported from GitLab, original author: @LiamClark
Mockito now ships an extension as support for JUnit 5.
We should add this as a dependency to use for students. This also allows us to finally drops the dependency on JUnit 4.
Removing JUnit 4 from the project will eliminate a set of errors, due to mixing JUnit 4 and JUnit 5 imports.