[3.15] gh-69134: Harden tkinter GUI tests that depend on a mapped widget (GH-152499...

[3.15] gh-69134: Harden tkinter GUI tests that depend on a mapped widget (GH-152499) (GH-152516)

Add wait_until_mapped() and AbstractTkTest.require_mapped() to
test_tkinter.support and use them to guard the assertions that need a
widget to be actually mapped (winfo_width(), identify(), coords(), ...).
This avoids intermittent failures under window managers that do not map
the widget promptly, without skipping the unrelated checks.
(cherry picked from commit 0fff6bd86cf0224152c509e295d3cbbd209098f3)

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