Implement example: functions_pubsub_unit_test

[coryan]

Line numbers are approximate:

https://github.com/GoogleCloudPlatform/golang-samples/blob/master/functions/helloworld/hello_pubsub_test.go#L15-L60

https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/functions/helloworld/sample_pubsub_test.py#L15-L45

[coryan]

Closed in #194