Add unit test for nodes with parent ids.

[faubulous]

When multiple inheritance is used, it is not sufficient that the node id just contains the URI of the resource. VS Code will throw an error when trying to insert a node with the same id multiple times in the tree.

The problem is a risk for the user experience and to prevent regressions in future versions there should be a unit test for this: the parent id should always be included in the node id.

As a side effect that would make the implementation of the the getParent method much easier as the URI is already encoded in the id.