Better assert message for documentation tests (#6575)

Author: Pierre-Sassoulas

doc/test_messages_documentation.py

@@ -146,9 +146,11 @@ def _runTest(self) -> None:
         expected_messages = self._get_expected()
         actual_messages = self._get_actual()
         if self.is_good_test_file():
-            assert actual_messages.total() == 0  # type: ignore[attr-defined]
+            msg = "There should be no warning raised for 'good.py'"
+            assert actual_messages.total() == 0, msg  # type: ignore[attr-defined]
         if self.is_bad_test_file():
-            assert actual_messages.total() > 0  # type: ignore[attr-defined]
+            msg = "There should be at least one warning raised for 'bad.py'"
+            assert actual_messages.total() > 0, msg  # type: ignore[attr-defined]
         assert expected_messages == actual_messages