Use pytest-xdist to parallelize tests

[bdice]

Description

This is an experiment to parallelize RMM tests. Now that the async MR doesn't prime itself, there should be fewer issues with this.

I may need to tweak some pool MR sizes.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

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[bdice]

/ok to test