Fix a CI flake (#94)

We need to consume the `Failure` in `test_error`, otherwise subsequent
tests will fail if the garbage collector runs during them.

Signed-off-by: Sean Quah <seanq@matrix.org>

test_s3.py

@@ -125,8 +125,7 @@ def test_error(self):
         self.body.error(excp)
         self.wait_for_thread()
 
-        self.assertTrue(deferred.called)
-        self.assertIsInstance(deferred.result, Failure)
+        self.failureResultOf(deferred, Exception)
 
     def wait_for_thread(self):
         """Wait for something to call `callFromThread` and advance reactor