…edTest.test_bounce) (apache#9673)

In Python 3, `filter` functions return iterators rather than `list` so it can traverse only once. Hence, the following loop will only see "empty" and then validation fails.

```python
        src_messages = self.source.committed_messages() # return iterator
        sink_messages = self.sink.flushed_messages()) # return iterator
        for task in range(num_tasks):
            # only first task can "see" the result. following tasks see empty result
            src_seqnos = [msg['seqno'] for msg in src_messages if msg['task'] == task]
```

Reference: https://portingguide.readthedocs.io/en/latest/iterators.html#new-behavior-of-map-and-filter.

Reviewers: Jason Gustafson <[email protected]>