Add gpu marker and test both Classic/dask-expr Dask DataFrames

[hoxbro]

By having it as markers it should be easy to test --gpu tests separately and not wait for the whole suite to pass. Running the non-GPU test continuously runs on the CI.

The classic / dask-expr was mostly something I wanted to test out if I could get to work, and was "easy" to do when the rest of the test infrastructure was updated.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.52850% with 26 lines in your changes are missing coverage. Please review.

Project coverage is 90.31%. Comparing base (0cdbd43) to head (751095d).

Files	Patch %	Lines
datashader/tests/test_dask.py	78.87%	15 Missing :warning:
datashader/tests/test_pandas.py	88.37%	5 Missing :warning:
datashader/tests/test_transfer_functions.py	84.37%	5 Missing :warning:
datashader/tests/test_xarray.py	87.50%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1341 +/- ## ========================================== - Coverage 90.38% 90.31% -0.08% ========================================== Files 91 92 +1 Lines 18764 18579 -185 ========================================== - Hits 16960 16779 -181 + Misses 1804 1800 -4 ```

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