Replace TimeoutTimer with CallbackTimer in GuiTestAssistant

[mdickinson]

In the wx version of the GuiTestAssistant we're writing our own TimeoutTimer class. We should be using the Pyface provided CallbackTimer instead, but it's currently causing segfaults (enthought/pyface#815). Once that's fixed upstream, we should replace the instantiation of TimeoutTimer with something like

timeout_timer = CallbackTimer(
    interval=timeout,
    repeat=1,
    callback=lambda: wx_app.exit(1),
)