Better unit tests

[rklaehn]

The current unit tests implicitly rely on the tree structure. E.g. the retain2 unit test, copied from a failed property based test, will only fail if you have a certain tree structure. But as soon as you would change the debug_fast options, you would no longer get the right tree structure for the test to be meaningful.

What should be done instead is to have a way to explicitly create a certain tree structure, or load the test trees from blocks. That way the tests would be more stable.