Convert the mocks for lab unit tests from Python dicts to JSON files. This makes the mocking of lab unit tests more consistent with how unit tests for most other modules are mocked. This also avoids unnecessarily mocking entire module functions, as only the HTTP requests made by those functions need to be mocked. Since the mocks are only supposed to contain JSON HTTP responses anyway, it makes more sense to keep them as JSON.

I placed the new JSON mocks in the tests/samples/ directory rather than the tests/mocks/ directory where the current mocks are located. I'm doing this as part of a larger effort to migrate all the mocks in tests/mocks/ to tests/samples so that mocks won't be split across two sibling directories.