chore(dataflow): update conftest, make publisher subprocess a daemon (GoogleCloudPlatform#6475)

* update conftest, make publisher subprocess a daemon

* drain pipeline when stopping

* fix renamed function

* test only on Python 3.8 since it uses a Dockerfile

* accept drained state as final state

* wait 3 minutes

Co-authored-by: Leah E. Cole <[email protected]>

Author: davidcavazos