Is your feature request related to a problem? Please describe.
I have some functionality that uses Pub/Sub push subscriptions I would like to test. It does not appear that the pstest package currently delivers push subscriptions, although you can create them.

Describe the solution you'd like
Add support for delivering messages via push subscriptions to pstest.

Describe alternatives you've considered
The Pub/Sub emulator supports push subscriptions but is invoked separately and requires Java.

Additional context
I'd be happy to take a first pass at implementing this if you'll accept a PR.