Extract test manager implementations

[oheger]

For its own tests, JGUIraffe has test implementations for the central builder interfaces, such as ComponentManager or WindowManager. These implementations can be useful for applications based on JGUIraffe as well, e.g. to help creating unit tests that can run without actually creating UI elements. Therefore, these implementations should be extracted into a separate artifact.

[oheger]

The project now publishes a jar with the test classes of the core module, which includes the desired test implementations. The implementations have been improved, so that they can be better used in application unit tests.