Good support for mocking responses, tests

[avivey]

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 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/ ?