[OTHER] Upgrade to JUnit 5

[tommens]

** Issue **
The current unit tests are written with JUnit 4. Upgrade all unit tests so that they are using the new JUnit 5 testing framework. Note that the syntax of JUnit 5 is different, so this requires changing some part of the code of the existing unit tests.

Make sure that the maven pom.xml file is updated so that all JUnit 5 tests are automatically executed (as is currently the case for the JUnit 4 tests.)

** Classification **
Upgrade of unit testing framework.

[tommens]

Resolved with commit cfc87da