Since the `extruded_shallow_water` [demo](https://github.com/firedrakeproject/firedrake/tree/master/demos/extruded_shallow_water) is a Python file instead of an RST file it is not picked up by the [glob](https://github.com/firedrakeproject/firedrake/blob/master/tests/demos/test_demos_run.py#L39) and so is never actually tested.
