[WIP] [POC] refact: coreg tests

[adebardo]

• [ ] Resolves #xxx,
• [ ] Tests added, otherwise issue #xxx opened,
• [ ] Fully documented, including api/*.md for new API,
• [ ] New optional dependencies added to both dev-environment.yml and setup.cfg,
• [ ] If contributor workflow (test, doc, linting) or Python version support changed, update CONTRIBUTING.md.