firestore: add a helper to streamline integration tests
firestore: add a helper to streamline integration tests

Change-Id: Ieefe14993e8c57d74f3f81b6e9925d7a618f086f
Reviewed-on: https://code-review.googlesource.com/25930
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

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