Document best practices for craft-application test fixtures

### What needs to get done

Write a how-to document that describes best practices for setting up test fixtures for an application and how to use them.

### Why it needs to get done

Apps do this in different ways and currently have fixtures that set things up in unexpected ways. We should make this uniform and the best way to do this is to start with a best-practices document.

No files or tests are named in the issue. Review how applications currently set up and use test fixtures, then document the recommended setup and usage patterns. Done means a how-to guide defines uniform best practices for application test fixtures.

Written by the indexing model from the issue text.