firestore: system tests take too long, timeout bulk release presubmits #16970

We have a hard limit of 3 hours for system tests to run before the jobs timeout.

I've been told the firestore system tests can take 1 hour+ to run.

On a recent bulk release PR, where firestore was being bumped, the presubmits timed out while running the firestore system tests.

The firestore system tests need to be sped up OR the test infra needs to be refactored to facilitate packages with longer system tests.