Added missing testing config

fixes pallets#1302

Author: accraze

docs/testing.rst

@@ -98,8 +98,10 @@ test method to our class, like this::
 
         def setUp(self):
             self.db_fd, flaskr.app.config['DATABASE'] = tempfile.mkstemp()
+            flaskr.app.config['TESTING'] = True
             self.app = flaskr.app.test_client()
-            flaskr.init_db()
+            with flaskr.app.app_context():
+                flaskr.init_db()
 
         def tearDown(self):
             os.close(self.db_fd)