TST: fix incorrect extension test (#39344)

Author: jbrockmendel

pandas/tests/extension/base/setitem.py

@@ -280,12 +280,11 @@ def test_setitem_frame_invalid_length(self, data):
         with pytest.raises(ValueError, match=xpr):
             df["B"] = data[:5]
 
-    @pytest.mark.xfail(reason="GH#20441: setitem on extension types.")
     def test_setitem_tuple_index(self, data):
-        s = pd.Series(data[:2], index=[(0, 0), (0, 1)])
-        expected = pd.Series(data.take([1, 1]), index=s.index)
-        s[(0, 1)] = data[1]
-        self.assert_series_equal(s, expected)
+        ser = pd.Series(data[:2], index=[(0, 0), (0, 1)])
+        expected = pd.Series(data.take([1, 1]), index=ser.index)
+        ser[(0, 0)] = data[1]
+        self.assert_series_equal(ser, expected)
 
     def test_setitem_slice(self, data, box_in_series):
         arr = data[:5].copy()