WARN: Avoid FutureWarnings in tests (pandas-dev#48398)

Avoid FutureWarnings in tests

Author: phofl

pandas/tests/frame/methods/test_update.py

@@ -136,7 +136,8 @@ def test_update_from_non_df(self):
     def test_update_datetime_tz(self):
         # GH 25807
         result = DataFrame([pd.Timestamp("2019", tz="UTC")])
-        result.update(result)
+        with tm.assert_produces_warning(FutureWarning):
+            result.update(result)
         expected = DataFrame([pd.Timestamp("2019", tz="UTC")])
         tm.assert_frame_equal(result, expected)