bpo-30780: Add IDLE configdialog tests (GH-3592)


Expose dialog buttons to test code and complete their test coverage.
Complete test coverage for highlights and keys tabs.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
(cherry picked from commit dd023ad1619b6f1ab313986e8953eea32c18f50c)

Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>
4 files changed