Add a couple of books for PHPUnit (#2628)

* Add couple of book for PHPUnit

* Added PDF text to the links

* Added PHPUnit Best Practices

* Alphabetically arranged

free-programming-books.md

@@ -1767,6 +1767,8 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 * [PHP Programming](https://en.wikibooks.org/wiki/PHP_Programming)
 * [PHP Security Guide](http://phpsec.org/projects/guide/)
 * [PHP: The Right Way](http://www.phptherightway.com)
+* [PHPUnit Best Practices](https://talks.thephp.cc/assets/2013-sunshinephp-phpunit-best-practices.pdf) - Sebastian Bergmann (PDF)
+* [PHPUnit Manual](https://phpunit.de/manual/current/en/phpunit-book.pdf) - Sebastian Bergmann (PDF)
 * [Practical PHP Programming](http://www.hackingwithphp.com)
 * [Practical PHP Testing](http://www.giorgiosironi.com/2009/12/practical-php-testing-is-here.html)
 * [Survive The Deep End: PHP Security](https://phpsecurity.readthedocs.org/en/latest/)