Adapt tests to upstream change

A test was broken by an upstream change.  This applies the fix that is also
used for the DNS plugins that are bundled with certbot:
certbot/certbot@6f27c32#diff-2fa7c8bfb2d0f68a9406030f098ff99840ae613ebc13bbabfe5aae3d0bd6356f

Author: peterthomassen

certbot_dns_desec/dns_desec_test.py

@@ -44,7 +44,8 @@ def setUp(self):
         # _get_desec_client | pylint: disable=protected-access
         self.auth._get_desec_client = mock.MagicMock(return_value=self.mock_client)
 
-    def test_perform(self):
+    @test_util.patch_get_utility()
+    def test_perform(self, unused_mock_get_utility):
         self.auth.perform([self.achall])
 
         self.mock_client.get_txt_rrset.assert_called_once_with(DOMAIN, "_acme-challenge")