How to test ReadFromPubSub

[Bergmeier, Andreas]

We have a PTransform, which uses ReadFromPubSub and now would like to test it.
Looking at 
https://github.com/apache/beam/blob/master/sdks/python/apache_beam/io/gcp/pubsub_test.py
 it seems like we would need something similar to TestReadFromPubSub.
I do however not understand how this works at all since e.g. there seems to 
appear a mock_pubsub out of thin air.
Can someone please give a rough sketch which parts need to work together how in 
order to test ReadFromPubSub in a TestPipeline?