A good place to start would be to look at the tests that come with Tk. If you have ever run "make test" against a Tk distribution, you will see various widgets appear on the display and disappear.

The main point is that tests for a Tk module won't actually be able to verify what the widgets actually look like, but should be able to check that they behave correctly. It is possible to fake mouse and keyboard events by using the pseudoevent mechanism.

Hope this helps