This commit adds a new fixture that emulates a S3 service in order to
improve the existing integration tests. This is very similar to what has
been made for Google Cloud Storage in #28788, and such tests would
have helped a lot to catch bugs like #22534.
The AmazonS3Fixture is brittle and only implements the very necessary
stuff for the S3 repository to work, but at least it works and can be
adapted for specific tests needs.