Adapt ttk tests to Tk 9.1

[serhiy-storchaka]

Several test_ttk tests fail against Tk 9.1 (currently 9.1b1) because of intentional changes in ttk::treeview and theming. The tests should be updated to accept the new behaviour:

• test_configure_selectmode — Tk 9.1 adds the single and multiple select modes.
• test_set — Treeview.set(item) now includes the tree column #0 in the returned dictionary.
• test_insert_item, test_selection — an item id now keeps the type of the passed value (e.g. int or bytes) instead of always being a string.
• test_heading_callback — a heading double click no longer performs the single-click action, so the test must not click twice at the same place.
• test_theme_create_image — the border element default width changed and is now display-scaled, so the exact requested width can no longer be asserted.

These are test-only changes; tkinter behaviour is unchanged.

(The related test_configure_selecttype failure is a separate Tk bug — -selecttype returns a one-element list instead of a scalar — fixed upstream, so no test change is needed here.)

Linked PRs

• gh-153380: Adapt ttk tests to Tk 9.1 changes #153381
• [3.15] gh-153380: Adapt ttk tests to Tk 9.1 changes (GH-153381) #153386
• [3.14] gh-153380: Adapt ttk tests to Tk 9.1 changes (GH-153381) #153387
• [3.13] gh-153380: Adapt ttk tests to Tk 9.1 changes (GH-153381) #153388