fix(sample): mitigate flakiness in subscriber_test (#304)
fix(sample): mitigate flakiness in subscriber_test (#304)

* fix(sample): mitigate flakiness in subscriber_test

fixes #289
fixes #288

I think there were few problems with the tests.

1. google.api_core.exceptions.Unknown is not retried.
2. number of messages published in test_receive_synchronously_with_lease
   was 5, but the sample code is fetching 3 messages in bulk.
3. assertion in test_receive_synchronously_with_lease was too strict.
4. failure in subscriber_test was incorrectly considered a failure in
   teardown of quickstart_test.

I hope these changes will mitigate the flakiness.

* lint

* also retry NotFound for dlq subscription

* lint

* ignore NotFound on deletion

* lint
googleapis/python-pubsub,googleapis/python-pubsub