Tests to prevent shipping same exception instance twice

[zachgoldstein]

So I see the notes about the global exception stack here:
https://github.com/airbrake/airbrake-python/blob/master/airbrake/notifier.py#L153

And later I see duplicate exception instances getting filtered out.
https://github.com/airbrake/airbrake-python/blob/master/airbrake/notifier.py#L173:

# dont ship the same exception instance twice
if exc_info[1] in self._exc_queue:
    exc_info = None, None, None
self._exc_queue.put(exc_info[1])

But I can't see any tests around this to make sure. This is probably not trivial, but it'd be nice to see the case where this is needed.