tests: migrate storage tests to pytest

• extract a bunch of initialization/testdata in a conftest.py
• add swh_indexer_storage and swh_indexer_storage_with_data fixtures
• 'explode' the main CommonTestStorage class into several pieces (one for every 'entity type' dedicated endpoints, eg. content_mimetype, ctags, etc. plus one for misc endpoints),
• get rid of the gen_generic_endpoint_tests() helper function, just write these test functions and inject them in endpoint_type specific test classes,
• use these classes + the fixture based client/server test framework from swh.core for test_api_client,
• use these classes for test_in_memory.