[3.15] gh-151678: Add tests for tkinter.Listbox (GH-151686) (GH-151712)

[3.15] gh-151678: Add tests for tkinter.Listbox (GH-151686) (GH-151712)

Cover previously-untested Listbox methods in ListboxTest: size, delete,
index resolution, nearest, see, activate, and the selection methods
(selection_set/clear/includes/anchor and their select_* aliases),
including the errors raised for invalid indices.
(cherry picked from commit cf3b3c11485a870d8e8c02579bed27a316838eb1)

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