Pub/Sub: update how to test with mock

[anguillanneuf]

Instead of rewriting the entire sub.py with a specified timeout, mock the subscriber client, the StreamingPullFuture returned by the subscribe method, as well as the result method on the future to get the desired effect.

Feel free to merge and delete the branch if approved!

This PR also includes the following changes:

• Use unique resource names (topics, subscriptions) to avoid test failures when more than one test are running at one time;
• Clean up resources after all the tests.

All formatting in this PR is taken care of by black.
TODO: Find out the new formatting standard from @GoogleCloudPlatform/python-samples-owners and update formatting in a following PR.

[leahecole]

Doesn't break the pubsub system tests so I'm happy :) . @tswast if you can take a peek at the mock stuff to make sure it looks okay, I'll merge once you give a green light.

[anguillanneuf]

All tests pass on my machine using nox. Not sure why they fail with Kokoro.

[leahecole]

Hi @anguillanneuf - I re-merged the branch and hopefully it doesn't break kokoro. If the kokoro pubsub tests are passing, we can double check that any other failures are unrelated to this PR. Once you address Tim's thing I'll take another look! :)

[gguuss]

I'm open to addressing the style issues in a separate PR so long as lint passes under flake, I'm re-running kokoro.

[gguuss]

LGTM when tests are green, thank you!

[gguuss]

@davidcavazos - Please squash and merge if you feel your comments have been addressed or can be resolved in a separate pull, I recommend the following commit message.

* Update test with mock
* Clean up resources after tests
* Use unique resource names avoid test failures
* Delete subscriptions in cleanup phase
* Ensure unique topic name
* Update assert to remove bytestring notation
* Rewrite PubSubToGCS test using dataflow testing module

[davidcavazos]

LGTM, thank you!

@gguuss I don't seem to have merge permissions, but everything looks good to me