We should handle TraitsUI helpers better in `GUITestAssistant`

[corranwebster]

This is awkward - should perhaps just duplicate code rather than trying to import it? See https://github.com/enthought/pyface/blob/32ac8b0fd79d828eaf2cdbe9a2b01a5a467677c4/pyface/ui/qt/util/gui_test_assistant.py#L42-L56