Factor AnonymousStorageServer fixture out for re-use

src/_zkapauthorizer/tests/fixtures.py

src/_zkapauthorizer/tests/test_base64.py

src/_zkapauthorizer/tests/test_client_resource.py

src/_zkapauthorizer/tests/test_controller.py

src/_zkapauthorizer/tests/test_matchers.py

src/_zkapauthorizer/tests/test_model.py

src/_zkapauthorizer/tests/test_plugin.py

src/_zkapauthorizer/tests/test_storage_protocol.py

src/_zkapauthorizer/tests/test_strategies.py