Need to make substitutions in app.yaml for local/remote testing on a per-test basis

[janbartel]

For testing sessions, I need to be able to put an env var into app.yaml (eg SESSIONS: 'datastore'). This env var shouldn't be there when local tests are run (unless the local docker execution is set up with the correct gcloud project properties etc). This substitution should really only be done when a specific test - eg RemoteSessionIntegerationTest - is run. I can't see how to accomplish this with the existing structure. Is there any way to accommodate this?