ci: run all of xarray's tests in downstream workflow

[maxrjones]

The test-downstream CI workflow didn't catch #3939 because it was running only zarr tests, rather than fsspec tests. This PR removes that filter.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.23%. Comparing base (6af3ea4) to head (12dbf1c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3940   +/-   ##
=======================================
  Coverage   93.23%   93.23%           
=======================================
  Files          87       87           
  Lines       11695    11695           
=======================================
  Hits        10904    10904           
  Misses        791      791           

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