Fix zero or negative UTC offset timezone test problem

A test was giving an incorrect control result via some assumptions about `datetime.timedelta`, which
only appear for zero or negative UTC offsets. This is due to the seconds and microseconds attributes
in `datetime.timedelta` being forced to be positive numbers. As the code comments say,

```
datetime.timedelta represents -21 microseconds as -1 day, +86399 seconds, +999979 microseconds
```

@matthewrmshin, please review.