Add custom required blocks comparison. Add feedback tests. Pull out common testing code.

1. For level-editor-defined "required blocks" sets, dynamically generate a required block test (1) using the exact XML as the display XML and (2) comparing `type` and `title` values for now
2. Add required block and block utility tests
3. Refactor out some common testing code
4. Add a few notes on testing and debugging tests to README