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,
• replace usage of the use_url hypothesis boolean statetgy by pytest.mark.parametrize fixtures; this allows to prevent from the need of resetting the storage, since tests are truly executed twice (thus with a new swh_storage),
• refactor test_db to use pytest-postgresql.

Disable (xfail) tests from test_snapshot.py, test_api_client
and test_in_memory for now.

Fixes/refactorings come with following revisions.