Improve test coverage of tkinter

[serhiy-storchaka]

Feature or enhancement

Large parts of the :mod:tkinter public API have little or no direct test
coverage. Widget configuration options are tested fairly thoroughly via the
AbstractWidgetTest framework in Lib/test/test_tkinter/test_widgets.py,
but many widget methods and the per-element configuration surfaces (tags,
embedded images and windows, marks, etc.) are not exercised at all.

For example, before this issue test_text.py tested only a handful of
Text methods (debug, search, search_all, count), leaving
indices, content manipulation, marks, tags, undo/redo, dump, embedded images
and windows, peers, geometry and the tag/image/window configuration options
untested.

The goal is to incrementally add tests for the untested tkinter API, widget by
widget and method by method:

• Text — methods and tag/image/window option configuration (in progress).
• Canvas — items, tags, coords, scanning, postscript, and item options.
• Listbox, Entry, Spinbox, Menu — selection, indices, item options.
• Misc/Wm — window manager, clipboard, selection, grab, event and
  winfo methods.
• Geometry managers (pack/place/grid) query methods.
• ttk widgets, Style, Treeview.

This complements the related audit of Tk commands that are only partially
wrapped by tkinter.

Links to previous discussion

Related to the recent tkinter documentation overhaul (gh-86726).

Linked PRs

• gh-151678: Add tests for tkinter.Text #151681
• gh-151678: Add tests for tkinter.Canvas #151683
• gh-151678: Add tests for tkinter.Menu #151685
• gh-151678: Add tests for tkinter.Listbox #151686
• gh-151678: Add tests for the remaining tkinter widgets #151687