[3.15] gh-153380: Adapt ttk tests to Tk 9.1 changes (GH-153381) (GH-153386)

[3.15] gh-153380: Adapt ttk tests to Tk 9.1 changes (GH-153381) (GH-153386)

- selectmode: new 'single' and 'multiple' modes
- set(): includes the tree column '#0'
- insert()/selection(): item ids keep the passed value's type
- heading_callback: avoid a double click on the second click
- theme_create_image: don't assert the exact (now scaled) width
(cherry picked from commit d6855c8ad00ffa2bee318b510d0e90573dddbbab)

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