[3.14] gh-151678: Add tests for tkinter.scrolledtext (GH-151753) (GH-151760)

[3.14] gh-151678: Add tests for tkinter.scrolledtext (GH-151753) (GH-151760)

Add a test for the ScrolledText widget, which had no tests: that it is
a Text widget held in a Frame with a Scrollbar, that Text methods work,
that the geometry manager methods are redirected to the frame while
configure is not, and that the scrollbar tracks the text view.
(cherry picked from commit a9db5cb52fefffc8fcdddc8e1c5f23222970825a)

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