Add PHPUnit Polyfills (#1301)

Repo: https://github.com/Yoast/PHPUnit-Polyfills/
Packagist: https://packagist.org/packages/yoast/phpunit-polyfills

Full disclosure: I'm the maintainer of the package. Yes, I did read the contributing guide, so I'll make my case ;-)

* Used by > [35K dependents](https://github.com/Yoast/PHPUnit-Polyfills/network/dependents)
* Over [20M downloads via Packagist](https://packagist.org/packages/yoast/phpunit-polyfills/stats)
* Used by [popular packages on the list](https://packagist.org/packages/yoast/phpunit-polyfills/dependents?order_by=downloads), including WordPress.
* [Recommended by the author of PHPUnit itself](https://phpunit.de/supported-versions.html) for running tests on multiple PHPUnit versions.

Co-authored-by: jrfnl <jrfnl@users.noreply.github.com>

README.md

@@ -330,6 +330,7 @@ Please see [CONTRIBUTING](https://github.com/ziadoz/awesome-php/blob/master/CONT
 * [PHPSpec](https://github.com/phpspec/phpspec) - A design by specification unit testing library.
 * [PHPT](https://qa.php.net/write-test.php) - A test tool used by PHP itself.
 * [PHPUnit](https://github.com/sebastianbergmann/phpunit) - A unit testing framework.
+* [PHPUnit Polyfills](https://github.com/Yoast/PHPUnit-Polyfills/) - Simplifies running PHPUnit tests on multiple PHPUnit versions.
 * [Prophecy](https://github.com/phpspec/prophecy) - A highly opinionated mocking framework.
 * [VFS Stream](https://github.com/bovigo/vfsStream) - A virtual filesystem stream wrapper for testing.