Streamline API testing

[amstilp]

Consider having methods of the AnVILAPIMockTestMixin that return or sets the expected json responses for various methods. That way, if the underlying API response changes, the tests are easier to update.

• Add methods to AnVILAPIMockTestMixin to get and/or set the expected json response for various methods.
• Modify test_models_anvil_api_unit to use these methods where appropriate.
• Modify test_views to use these methods where appropriate.