Depend on JUnit 4.12 instead of JUnit 5.5.1

[joschi]

Context

While JUnit 5.5.1 (API) is being declared as a dependency, the actual tests are all using JUnit 4.x.

Contributor Checklist

• Added relevant integration or unit tests to verify the changes
• Ensured that tests pass locally: mvn clean package
• Ensured that the code meets the current checkstyle coding style definition: mvn clean verify -Pcheckstyle -Dmaven.test.skip=true -B

[TheSnoozer]

Thanks for the contribution. Looks good!