Packages: offer standardized way to set up WordPress / run WordPress PHPUnit tests within a package

Until now, we've relied on 2 packages to help us with unit tests inside our PHP packages:
- https://github.com/Automattic/wordbless
- https://github.com/Brain-WP/BrainMonkey

While those 2 options work, they do not offer the full WordPress core PHPUnit test library that is included with WordPress' development environment:
https://github.com/WordPress/wordpress-develop/tree/master/tests/phpunit

That's sometimes problematic, for example when wanting to develop tests against functions that use `WP_Query` for example: #19579

It would be nice to come up with a solution that we can agree on, and that we would rely on for most packages. Maybe something like https://github.com/wp-phpunit/wp-phpunit

Start by comparing the existing wordbless and BrainMonkey approaches with the WordPress core PHPUnit library in wordpress-develop/tests/phpunit. Review the wp-phpunit option and the context in #19579 to identify package requirements and compatibility concerns. Done means documenting or implementing an agreed standardized way for packages to set up WordPress and run the full PHPUnit test library.

Written by the indexing model from the issue text.