MAINT: Use pytest-xdist in CI

[derb12]

Description

Just a test to see if running pytest-xdist with 4 processes runs fine or if it's too much for github actions. Running locally with act shows a time improvement even with the limited resources of one machine.

Type of Pull Request

• Bug Fix
• New Feature
• Miscellaneous Changes (refactor, code improvements, etc.)
• Documentation or Example Programs

Pull Request Checklist

• New code and/or documentation is valid for use with the BSD 3-clause license.
• New code is fully documented with docstrings that follow Numpy style,
  if applicable.
• New code follows PEP 8 standards as closely as possible, if applicable.
• Added/updated tests and ensured they pass locally, if applicable.
• Verified that documentation builds locally, if applicable.

[derb12]

Comparing to the last PR where actions ran against development in #84, affected workflows decreased by ~2-3 minutes for non free-threaded and ~4 minutes for py314t, so seems like it works fine.