add tests for keyword arguments passed to engines when opening a datatree

[JessicaS11]

Our in-progress test development from SciPy 2024 sprints. Hoping to get back to finishing implementation of these tests, but happy for someone else to run with them too (let me know and I can give you access to this fork). Relevant to #8572, motivated by #9199, which solved a bug introduced in #9014.

• [ ] Closes #xxxx
• [ ] Tests added
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst
• [ ] New functions/methods are listed in api.rst