Add demo tests

[thibaudcolas]

I wonder if it would be worthwhile for us to set up a few unit tests in the bakerydemo project? To demonstrate what we document is Testing your Wagtail site.

For core development purposes I assume there isn’t as much of a need here, as we can just as well test those utilities in Wagtail’s test suite. But for demo purposes, writing unit tests is an important capability of Django projects, so feels worthwhile to demonstrate the application of this to Wagtail (and in particular pages and StreamField).

[Scotchester]

I think that'd be helpful!

[Rish-it]

While we don’t need them for core Wagtail development, I think it’s still valuable to show how testing works with pages and StreamField. It should be useful for anyone using the demo as a reference.