[3.13] gh-151678: Add tests for tkinter.Text (GH-151681) (GH-151705) (GH-151715)

[3.13] gh-151678: Add tests for tkinter.Text (GH-151681) (GH-151705) (GH-151715)

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.
(cherry picked from commit 4ac809e10bdb413d7dd8c7bab7e365b2b618ad91)
(cherry picked from commit 22c000eeb4341f650f2c660b29d49f3f44741392)

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