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in_memory: fix handling of 'hidden' content in InMemoryStorage and update tests

Description

in_memory: fix handling of 'hidden' content in InMemoryStorage and update tests

This defines a local swh_storage fixture that uses an InMemoryStorage to
execute tests defined in classes TestStorage and TestStorageGeneratedData
(from swh.storage.tests.test_storage).

Adapt tests for the InMemoryStorage to new storage test infra.