IDLE: Standardize naming convention for DummyEditwin in tests (GH-13876) (#13885)

* Change from Dummy_Editwin to DummyEditwin to match other tests.
(cherry picked from commit 7f8a38a7c47823c17adab469fcb4f762f4e945b7)

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