tests: fix bogus test case

stanza.match worked accidentally, but the two elements used in that test
actually conflict with each other (not overriding the plugin_attrib,
which makes it then default to 'plugin', leading to shenanigans).

tests/test_stanza_element.py

@@ -456,6 +456,7 @@ class TestElementBase(SlixTest):
 
         class TestSubStanza(ElementBase):
             name = "sub"
+            plugin_attrib = name
             namespace = "baz"
             interfaces = {'attrib'}