add test examples to docs

[davidism]

Finished up the missing section, closes #6. Also address some notes from @pamelafox.

• Show an example of using the app context, call out that it shouldn't be pushed around requests.
• Show an example of an app context fixture and using it with pytest.mark.usefixtures for tests that don't make requests.
• Explain how the test_config argument to the app factory works, and point out that it's used in the app fixture.
• Clarify that the same db extension instance is imported in code and tests.

The one comment not addressed was where to define BaseModel. That's because it shouldn't matter. It's shown being defined in a models.py file in the getting started docs. But really, it and other models can be defined wherever, as long as they're importable. It's the same as any other code in the application.