tests: fix prerequisites for stanza tests

tests/test_stanza_xep_0060.py

@@ -3,10 +3,19 @@ from slixmpp.test import SlixTest
 import slixmpp.plugins.xep_0004 as xep_0004
 import slixmpp.plugins.xep_0060.stanza as pubsub
 from slixmpp.xmlstream.stanzabase import ET
+from slixmpp.xmlstream import register_stanza_plugin
 
 
 class TestPubsubStanzas(SlixTest):
 
+    def setUp(self):
+        register_stanza_plugin(
+            xep_0004.FormField, xep_0004.FieldOption, iterable=True
+        )
+        register_stanza_plugin(
+            xep_0004.Form, xep_0004.FormField, iterable=True
+        )
+
     def testAffiliations(self):
         "Testing iq/pubsub/affiliations/affiliation stanzas"
         iq = self.Iq()