Update testing.rst

Added installation instructions to avoid the problem

Author: vihuarar

testing.rst

@@ -50,7 +50,12 @@ Symfony application.
 
     Code coverage can be generated with the ``--coverage-*`` options, see the
     help information that is shown when using ``--help`` for more information.
-    To avoid the error "No code coverage driver is available" when you are generating code coverage in html you should install php-debug 
+    To avoid the error "No code coverage driver is available" when you are generating code coverage you must install (xdebug, pcov,    phpdbg)
+
+To install x-debug type on Ubuntu: 
+.. code-block:: terminal
+    $ sudo apt-get install php-xdebug
+    
    
 .. index::
    single: Tests; Unit tests