Provide helpers to write unit tests for Optuna samplers

[c-bata]

Currently, OptunaHub does not provide utilities to write unit tests for samplers. So some OptunaHub packages copy-and-paste the content of Optuna's test_samplers.py. Please check out the following files for details.

• https://github.com/optuna/optunahub-registry/blob/main/package/samplers/ctpe/tests/test_sampler.py
• https://github.com/optuna/optunahub-registry/blob/main/package/samplers/smac_sampler/tests/test_sampler.py

This issue proposes to provide utilities functions for checking these test scenarios and making unit tests DRY.