Parallelize the test suite and fix a test polluted bug

[hoxbro]

I see this flaky test: datashader/tests/test_quadmesh.py::test_raster_quadmesh_autorange_reversed[dask.array] Not flaky at all: pytest 'datashader/tests/test_quadmesh.py::test_raster_quadmesh_autorange_reversed[dask.array]' as this also fails. It is test pollution from the other array...

Want to merge it after my other prs.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.11765% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 85.63%. Comparing base (1bf733e) to head (79b5883). Report is 1 commits behind head on main.

:exclamation: Current head 79b5883 differs from pull request most recent head 65f146b

Please upload reports for the commit 65f146b to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1338 +/- ## ========================================== + Coverage 85.57% 85.63% +0.06% ========================================== Files 51 53 +2 Lines 11296 11316 +20 ========================================== + Hits 9667 9691 +24 + Misses 1629 1625 -4 ```

[hoxbro]

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