Provide examples for ModalDialogTester and GuiTestAssistant

I have not been able to find any examples for [`ModalDialogTester`](https://github.com/enthought/pyface/blob/master/pyface/ui/qt4/util/modal_dialog_tester.py) or the [`GuiTestAssistant`](https://github.com/enthought/pyface/blob/master/pyface/ui/qt4/util/gui_test_assistant.py). The code itself is well documented and easy-ish to understand but how the various methods can be used for testing is not clear.

Note that the classes themselves cannot be found in the docs page.

A good place to start is the tests for `ModalDialogTester` and `GuiTestAssistant` which provide a few examples of how the different methods on the classes can be used.