We should explain how tests should be organized to avoid cross-library imports.

• The Cupertino tests README.
• The Cupertino Material test directory, along with its README should be refactored and deleted.
• A README should be added to test/widgets and test/material.

The general principles to explain are given in the design doc (flutter.dev/go/decoupling-framework-tests), summarized here:

• The Cupertino library/tests should never import Material. It should test its widgets under a full-Cupertino scenario.
• The Material library should test its widgets in a full-Material scenario.
• Design languages are responsible for testing their interoperability with Widgets.
• Tests that cover interoperability between Material and Cupertino should go in Material. See the next section for when and how interoperability should be tested.
• The Widgets library/tests should never import Cupertino or Material.

Part of #177028