Add `test-gpu` task

[hoxbro]

This should make it easier to test CuDF (locally). I'm using it as a marker as I only want to be able to run the GPU tests if this task is called.

I have used the versions from https://github.com/rapidsai/cudf?tab=readme-ov-file#conda

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 34.00000% with 66 lines in your changes are missing coverage. Please review.

Project coverage is 88.18%. Comparing base (bc684e3) to head (49ddb0b). Report is 3 commits behind head on main.

Files	Patch %	Lines
holoviews/element/selection.py	9.09%	20 Missing :warning:
holoviews/tests/operation/test_operation.py	35.71%	18 Missing :warning:
holoviews/tests/element/test_selection.py	26.66%	11 Missing :warning:
holoviews/tests/core/data/test_cudfinterface.py	46.66%	8 Missing :warning:
holoviews/core/data/cudf.py	0.00%	5 Missing :warning:
holoviews/tests/operation/test_datashader.py	20.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6217 +/- ## ========================================== - Coverage 88.21% 88.18% -0.04% ========================================== Files 321 321 Lines 67289 67360 +71 ========================================== + Hits 59361 59402 +41 - Misses 7928 7958 +30 ```

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[hoxbro]

Also added support for _select_mask_neighbor as the test was easy to run.