Update how we run PHPUnit

[dkotter]

Description of the Change

Our PHPUnit workflow has started to fail and in investigating that, seems GitHub has updated their default version of Docker that is installed in their runners and that is conflicting with the tool we use to run our tests. That tool hasn't been updated since June 2024 so I'm not confident they'll push out a fix.

Also, there's a better way to run PHPUnit tests now that didn't exist/we didn't know about when we set this up (using wp-env) so this is a good opportunity to clean up that setup.

The following updates are made:

• We update our GitHub Action workflow to use the wp-env environment to run our PHPUnit tests (and run those on a wider matrix)
• Update our various config and bootstrap files as needed
• Migrate off of WP Mock and just use the WordPress testing suite

Note most of this was copied from the approach we took on ClassifAI.

How to test the Change

Ensure the PHPUnit GitHub Action runs and passes on this PR

Changelog Entry

Developer - Update our PHPUnit workflow

Credits

Props @dkotter

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.