[3.15] gh-151678: Add tests for tkinter widget virtual events (GH-151793) (GH-151805)

[3.15] gh-151678: Add tests for tkinter widget virtual events (GH-151793) (GH-151805)

Verify the virtual events that widgets emit in response to user
interaction, driven by generated events: <<ListboxSelect>> (Listbox),
<<Increment>> and <<Decrement>> (ttk Spinbox), and <<TreeviewSelect>>,
<<TreeviewOpen>> and <<TreeviewClose>> (ttk Treeview).
(cherry picked from commit e51b616efff845ea2a7d312aa43d5f5100064d88)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>