fix(tests): group mem heavy tests together

[SamWilsn]

🗒️ Description

Some tests make heavy use of lru_cache. Others are just memory hungry. By grouping these tests onto a single xdist worker, we consolidate caches and prevent multiple big-memory tests from running at the same time.

In some very rough benchmarks, I see a reduction in memory use of about 1.2GB on my local machine.

✅ Checklist

• All: Ran fast tox checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  uvx --with=tox-uv tox -e lint,typecheck,spellcheck,markdownlint

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered adding an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).