Great start! Some revisions:
* Create root directly (and just once); add root.destroy, which added several warnings.
* Create Text directly; test should not require Editors. This removed warnings. I suspect that EditorWindow.py does other things that are not properly undone. In any case, tests should only create needed objects.
* Worked on docstrings; polished _decode.
* Revised .insert to handle '' and 'a....z\n' and added corresponding tests.
* Consistently used 'end' instead of 'end' here and 'END' there.
* Added test_no_delete and edited mock delete to pass.
* Finished compare methods and added tests.
* Deleted non-tk .setDate/.getData, which only slightly abbreviate the real .get and .insert methods. I notice that Phil already changed the FormatParagraph test to not use them.

Here is the result. I think it is about ready to commit after sleeping on it and a final review.