Spinoff of #27732, which changed 5 tested bell-using classes to consistently call 'self.bell()' (instead of self.text.bell or whatever) so tests for such classes could replace the function with a quiet function.  For 4 of the classes, the replacement is currently 'lambda: None'.  For undo, it is a quiet mock that replaced a noisy mock wrapping and calling the original bell().  The purpose of the mock, either way, is to test that bell() is called when intended.

I plan to add a new idle_test.mock_tk.bell function.  I am considering having it automatically reset 'called' when 'called is accessed.  In any case, this issue is about extending the use of mock bells to other tests.