Adds dask-expr to test dependency

[skrawcz]

So that tests don't fail -- dask now needs it as
a dependency.

Also turning using dask-expr off for tests.

Changes

• requirements-test.txt
• requirements-docs.txt
• h_dask integration test conftest.py

How I tested this

• locally

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.