Testing Tools: add PHPUnit Polyfills

The [PHPUnit Polyfills](https://github.com/Yoast/PHPUnit-Polyfills) were created to allow devs to write tests for the latest version of PHPUnit, while still allowing those tests to run on older PHPUnit versions.

Full disclosure: I'm the author of the library.

Note: this library is the [endorsed](https://phpunit.de/supported-versions.html) by the author of PHPUnit as the way to support PHPUnit cross-version compatible tests.

Author: jrfnl

_posts/11-04-01-Complementary-Testing-Tools.md

@@ -16,6 +16,7 @@ libraries useful for any preferred approach taken.
 [PHPSpec] and can be used with [PHPUnit].
 * [php-mock]  is a library to help to mock PHP native functions.
 * [Infection] is a PHP implementation of [Mutation Testing] to help to measure the effectiveness of your tests.
+* [PHPUnit Polyfills] is a library that allows for creating PHPUnit cross-version compatible tests when a test suite needs to run against a range of PHPUnit versions.
 
 
 [Selenium]: https://www.seleniumhq.org/
@@ -27,3 +28,4 @@ libraries useful for any preferred approach taken.
 [php-mock]: https://github.com/php-mock/php-mock
 [Infection]: https://github.com/infection/infection
 [Mutation Testing]: https://en.wikipedia.org/wiki/Mutation_testing
+[PHPUnit Polyfills]: https://github.com/Yoast/PHPUnit-Polyfills