gh-151678: Add tests for tkinter.Text (GH-151681)

Cover previously-untested Text methods (indices, content, marks, tags,
undo/redo, dump, embedded images and windows, peers, and geometry) and
the tag, embedded-image and embedded-window configuration options.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka