Update testing.rst

added a comment to a requirement to generate code coverage in html

Author: vihuarar

testing.rst

@@ -50,7 +50,8 @@ 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 
+   
 .. index::
    single: Tests; Unit tests