Motivation

Third-party storage implementations currently have to copy Optuna’s storage tests (which is written in tests/storage_tests/test_storage.py). By exposing the shared StorageTestCase in optuna.testing, third-party Optuna storages can run the same test suites, improving consistency and fixing compatibility issues without duplicating test logic.

Description of the changes

This PR suggests to exposing StorageTestCase, so that third-party Optuna storages can write tests like below:

import pytest
from optuna.testing.pytest_storages import StorageTestCase
from optuna_integration.dask import DaskStorage


@pytest.fixture
def storage() -> BaseStorage:
    return DaskStorage(...)

class TestDaskStorageCase(StorageTestCase):
     @pytest.mark.skip("DaskStorage does not support this feature")
     def test_pickle(self, storage: BaseStorage) -> None:
        ...