Ensure that the XEP-0086 plugin is loaded.

Since the XEP-0086 plugin auto adds error code values,
it must be reliably loaded or unloaded when certain tests
are run so that stanzas may be matched. In this case, we
ensure that the plugin is used.

tests/test_stanza_error.py

@@ -3,6 +3,11 @@ from sleekxmpp.test import *
 
 class TestErrorStanzas(SleekTest):
 
+    def setUp(self):
+        # Ensure that the XEP-0086 plugin has been loaded.
+        self.stream_start()
+        self.stream_close()
+
     def testSetup(self):
         """Test setting initial values in error stanza."""
         msg = self.Message()