fix error namespace for ComponentXMPP

in SlixTest, if mode=="component", restore
jabber:client namespace afterwards

Fixes #3476
Fixes #3474

tests/test_stream_exceptions.py

@@ -10,9 +10,6 @@ class TestStreamExceptions(SlixTest):
     Test handling roster updates.
     """
 
-    def tearDown(self):
-        self.stream_close()
-
     def testExceptionContinueWorking(self):
         """Test that Slixmpp continues to respond after an XMPPError is raised."""