Update testing.rst
Update testing.rst

https://github.com/silexphp/Silex-Skeleton/blob/master/phpunit.xml.dist

Not adding bootstrap segment to phpunit.xml.dist causes Fatal Error: Class 'Silex\WebTestCase' not found
gencer/Silex,fabpot/Silex,fabpot/Silex,silexphp/Silex,romainneutron/Silex,romainneutron/Silex,romainneutron/Silex