Better unit tests

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.

Locate the retain2 unit test and inspect how debug_fast options determine the tree structure used by the test. Compare the current test setup with the proposed approaches of explicitly creating a tree or loading test trees from blocks. Done means the tests no longer depend implicitly on debug_fast or incidental tree structure and remain meaningful after those options change.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.