[test] Setup S3Storage in a hook

Instead of doing storage attribution in test setup, we implement a test
hook (in test/data/hook.py) as would be done in real application. As a
consequence, we change the way s3 requests are mocked (using moto).
Namely, we introduce a "_s3_client" class method in S3Storage that is
mocked in test setup so that mocking operates in the test hook.

In migration tests, instead of instantiating a new storage object, we
reuse the one bound to Image's data attribute so as to benefit from the
active mock.