[3.15] gh-151678: Add tests for tkinter.Menu (GH-151685) (GH-151709)

[3.15] gh-151678: Add tests for tkinter.Menu (GH-151685) (GH-151709)

Cover previously-untested Menu methods in MenuTest: adding, inserting and
deleting items of every type, index resolution, invoking items, entry x/y
positions, and post/unpost/tk_popup mapping.

Also test per-entry configuration options and the errors raised for
invalid indices, entry types, option names and option values.
(cherry picked from commit ef5c32a40be50a33a9b7ac39ee64e6893bc22f60)

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