use app.testing property instead of config

docs/testing.rst

@@ -41,7 +41,7 @@ In order to test the application, we add a second module
 
         def setUp(self):
             self.db_fd, flaskr.app.config['DATABASE'] = tempfile.mkstemp()
-            flaskr.app.config['TESTING'] = True
+            flaskr.app.testing = True
             self.app = flaskr.app.test_client()
             with flaskr.app.app_context():
                 flaskr.init_db()
@@ -98,7 +98,7 @@ test method to our class, like this::
 
         def setUp(self):
             self.db_fd, flaskr.app.config['DATABASE'] = tempfile.mkstemp()
-            flaskr.app.config['TESTING'] = True
+            flaskr.app.testing = True
             self.app = flaskr.app.test_client()
             with flaskr.app.app_context():
                 flaskr.init_db()