[3.15] gh-151678: Add tests for the remaining tkinter widgets (GH-151687) (GH-151725)

[3.15] gh-151678: Add tests for the remaining tkinter widgets (GH-151687) (GH-151725)

Cover previously-untested methods of several widgets:

* Button, Checkbutton and Radiobutton: invoke, flash and toggle;
* Entry: delete, icursor and the select_* aliases;
* Spinbox: invoke, identify and scan;
* Scale and Scrollbar: identify, and Scrollbar fraction and delta;
* PanedWindow: panes, remove/forget, sash and proxy positioning,
  identify, and adding panes with configuration options.

Also test that invoke does nothing for a disabled button and the
errors raised for invalid indices, coordinates, option names and values.
(cherry picked from commit 93b9e7666f4337e3cacfed6993568e4bec575e9b)

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