'Find in files' by replacing 'open with implicit close' by 'with open' in
GrepDialog method grep_it. Streamline code with enumerate(), direct file
iteration, and output tweak. Add test for this method, including output format.
Fix bug in existing human test and add instructions; fix two bugs in tested
code; remove redundancies, add spaces, and change two internal method names.
Add mock_tk with mocks for tkinter.Variable subclasses and tkinter.messagebox.
Use mocks in test_config_name to unittest methods that are otherwise gui-free.
Terry Jan Reedy