Add pytest-xdist for parallel test execution

[rg936672]

PR Type

• CI related changes
• Tests

Description

Add pytest-xdist to allow tests to execute in parallel. This should speed up CI.

How Has This Been Tested?

Existing tests pass as expected. CI is marginally faster.

Does this PR introduce a breaking change?

No.

Checklist before requesting a review

• I have made sure that my PR is not a duplicate.
• My code follows the style guidelines of this project.
• I have ensured my code is easy to understand, including docstrings and comments where necessary.
• I have performed a self-review of my code.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• New and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.
• I have updated CHANGELOG.md, if appropriate.

[github-actions]

Performance review

Commit 1d78dd3 - Merge 01e64d4 into 94a1a2d

No significant changes to performance.