Run system test suite on emulator as well as live service

[crwilcox]

Some of our libraries have emulator support and it would be a nice to have if the emulator was used to run our system test suites.

I think starting with bigtable or firestore would make sense, then we could move to others if desired.

• Firestore
• Bigtable
• Datastore

[tseaver]

Datastore is the "oldest" one in the repository:

https://github.com/googleapis/google-cloud-python/blob/951b9c68d2390eedd271b7eb1ca363cb48d82d8d/datastore/google/cloud/datastore/client.py#L245-L249