Add PHPT to Tests section (#648)

README.md

 * [Phake](https://github.com/mlively/Phake) - Another mock object library for testing.
 * [Pho](https://github.com/danielstjules/pho) - Another behaviour driven development testing framework.
 * [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.
 * [Prophecy](https://github.com/phpspec/prophecy) - A highly opinionated mocking framework.
 * [Samsui](https://github.com/mauris/samsui) - Another fake data generator library.