Merge: Mock Github API tests

This PR adds a mock to GithubAPI so we can avoid sending requests to the API on CI.

For each API call we save the actual Github response body from the API and reuse it during the tests.
The attribute `update_responses_cache` can be set to `true` so the cache files are updated from the API when `nitunit` is called.

Pull-Request: #2753
Reviewed-by: Jean Privat <jean@pryen.org>