TST: Use (unused) window parameter of test_freq_window_not_implemented
Realized that `window` parameter is unused so this is fixing it. Calling `sum` function as the exception is only raised when a function is called with `VariableOffsetWindowIndexer`. Not sure if I need to add an entry to whatsnew for this

- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks @ihsansecer 