tests: refactor main storage tests

• use pytest instead of unittest.TestCase plumbing
• extract data from the TestStorageData into a data storage_data module; this module also provide a simple helper StorageData class that mimics the original class (access by attributes),
• implement a series of pytest fixtures for these storage specific tests,
• get rid of most hypothesis-based tests.

---

Migrated from D2090 (view on Phabricator)