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

doc/testing.rst

@@ -192,6 +192,7 @@ look like this:
              processIsolation="false"
              stopOnFailure="false"
              syntaxCheck="false"
+             bootstrap="vendor/autoload.php"
     >
         <testsuites>
             <testsuite name="YourApp Test Suite">