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/ ?

没有指定文件或测试。首先检查仓库中现有的 GraphQL 客户端测试以及任何 response-mocking 支持，然后将所请求的行为映射到可测试的接口。完成的标准是：可以根据 queries 和 schemas 检查 mocks，支持 Don't-care 值和 sentinel 值，允许按测试进行 patching，并明确是否包含自动生成。

由索引模型根据 Issue 内容生成。