Add TestCase that automatically test PageFactory subclasses

[ababic]

Dependant on #62
Resolves #45

PageAutoTestCase is an importable test case that provides an instantly high level of coverage for custom page types in a standard (non-headless) Wagtail project.

All developers need to do is:

• Import PageAutoTestCase into a module where their test runner will pick it up
• For each page type they want to test, define a PageFactory capable of creating fully functional instances

The edit-ability, serve-ability (for multiple routes), and preview-ability (for all preview modes) should then be tested for each type.

Developers can add autotest=False to the Meta class to exclude factories from these tests. For example,
where a factory is created as a 'base' for a series of subclasses, and isn't intended to be used directly.

What if my page types aren't a good fit?

If none of them are, simply don't import PageAutoTestCase into your project, and the tests should not run as part of your test suite.

If only a few page types are incompatible with PageAutoTestCase, you may wish to add autotest=False to the Meta class for those factories to exclude them.

Can I customise PageAutoTestCase?

PageAutoTestCase isn't designed to be subclassed. However, if you wanted to define something similar,, you could subclass PageFeaturesTestCase, which would give you access to the following assertions:

• assertCanEditPageWithoutErrors(page: Page)
• assertCanRenderPageWithoutErrors(page: Page, route_path: str = "/")
• assertCanPreviewPageWithoutErrors(page: Page, preview_mode: str = "")