app_test.py: Use coroutine version of fetch

Avoid tornado getting confused and stopping/starting the ioloop
incorrectly

wpull/app_test.py

@@ -87,7 +87,7 @@ def test_one_page(self):
             exit_code = yield app.run()
             self.assertTrue(os.path.exists('index.html'))
 
-            response = self.fetch('/')
+            response = yield self.http_client.fetch(self.get_url('/'))
 
             with open('index.html', 'rb') as in_file:
                 self.assertEqual(response.body, in_file.read())