Add / Improve tests for tree goal

[mthmulders]

The Tree goal lacks decent tests. This makes it hard to add new features without breaking stuff (see e.g. the discussion in #24). I decided to re-write tests for this goal by mocking away the components that the Tree goal uses.

The work is far from done. That's why there's no JIRA issue yet, and I deliberately did not check any of the licensing checkbox at the end of the checklist. Rather, I'd have a discussion whether this is the right approach for testing the goal. If so, I can continue the work in this direction. If all is done, we can do the administration (JIRA, agreement of Apache License, etc.).

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