[3.7] bpo-30780: Add IDLE configdialog tests (GH-3592) by miss-islington · Pull Request #18218 · python/cpython

@csabella @miss-islington

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 dd023ad)

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

terryjreedy