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.

[hoxbro]

I see no reason why to test for 1 to 4, so I removed 3.

[codecov]

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 ⚠️
datashader/tests/test_pandas.py	88.37%	5 Missing ⚠️
datashader/tests/test_transfer_functions.py	84.37%	5 Missing ⚠️
datashader/tests/test_xarray.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            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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