Join the PHPUnit test cleanup for 7.2

WordPress core runs about 31,000 PHPUnit tests on every commit. These tests need maintenance just like core code: many leak state into other tests, some assert nothing, and several are coupled to network calls.

A goal for the Build/Test Tools component for the 7.2 release is to reduce build time and load across wordpress-develop and related repos. A healthier test suite will help!

This post is intended to show the pieces any contributor can claim, the same way the JSHint effort did in 2013. Same model is running today for 7.2 in PHPStan (#65817), docblocks (#65860), TypeScript (#65997), and JSDoc