Integration tests for PubsubIO

PubsubIOTest doesn't contain a single test that reads or writes a Pubsub topic. As far as I can tell, the public Beam repo doesn't have any tests that do that, either. So, it's really easy to break Pubsub. It's particularly easy to break because Dataflow runner has a special implementation of Pubsub.

We should have tests for PubsubIO (read and write; with and without attributes); running against direct runner and against Dataflow runner (since Dataflow runner has a custom implementation of PubsubIO)

Imported from Jira [BEAM-2108](https://issues.apache.org/jira/browse/BEAM-2108). Original Jira may contain additional context.
Reported by: jkff.

Start with PubsubIOTest and inspect how PubsubIO is currently exercised, then compare the direct runner and Dataflow runner paths mentioned in the issue. Done means covering Pubsub topic reads and writes, both with and without attributes, against both runners.

Written by the indexing model from the issue text.