Add unit tests for version classes

This includes adding Maven test infrastructure such as
  - Maven Surefire Plugin,
  - JUnit 5 Jupiter,
  - Mockito.

The plugins and test dependencies are only version-managed in the parent
POM and to be added to each module requiring them. Working the Maven
Way, we want to avoid adding plugins and dependencies on top level, if
they are not needed everywhere.

There are three test classes, mirroring the version classes under test.
The tests showcase
  - basic unit testing,
  - JUnit Jupiter parametrised tests (incl. human-readable test names),
  - using Mockito to mock static methods.
7 files changed