Good support for mocking responses, tests

I think this should be high in the roadmap:
Make it easy/fun for application writers to write mock gql responses, to support their tests.

This includes:
- Check the mock used in the test matches the query & schema used.
- Allow a mocked response to contain "don't-care" values (which the test writer knows aren't important for the specific test) and [sentinel](https://docs.python.org/3/library/unittest.mock.html#unittest.mock.sentinel) values (which the author will want to check for identity).
- Allow a basic mocked response to be written for a test class, and then 'patched" in place for each test with specific changes.
- Maybe automagically generate a mocked response, à la https://github.com/marak/Faker.js/ ?