firestore: Fix broken test.
firestore: Fix broken test.

Change-Id: Ifa85ba2e8b15f72e44ee0e04129235efb6bfd358
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/56131
Reviewed-by: Tyler Bui-Palsulich <b41e4aa57d2ad7eb1b29835a1b483e209dcc3c0e@google.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go