CDAP-488 Improve integration tests for python

This block is confusing to read. We can precompute the expected values, the received values, and do an assert on the sets.

```python
self.assertTrue(len(received_events) == 50)
expected_events = [msg + str(i) for i in xrange(50)]
received_event_bodies = [event['body'] for event in received_events]
self.assertTrue(set(expected_events) == set(received_events))
```