firestore: TestIntegration_EnterpriseDB failed

Note: #20040 was also for this test, but it was closed more than 10 days ago. So, I didn't mark it flaky. ---- commit: 637505e5d5f090eec020604c3702b6ace11dcd38 buildURL: [Build Status](https://source.cloud.google.com/results/invocations/fc4d9bd0-87c0-4f05-bc79-7a8287fd3525), [Sponge](http://sponge2/fc4d9bd0-87c0-4f05-bc79-7a8287fd3525) status: failed