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This option is misleading as one would expect S3Storage to enable
deletion by default.

Hooks require uploaded files to be accessible and open

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.

We now use an "Image" entity type defined in test/data/schema.py with a
"data" bytes attribute along with a "thumbnail" one. The former is used
as File's data attribute was. The latter is now used within
S3StorageMigrationTC (instead of "data" attribute); this is in
preparation of the next changeset in which s3 storage setup will be
handled in a test hook (instead of test setup).

So as to ease testing in downstream projects.

This seems more appropriate as we're actually not doing any database
setup.