firestore: increase time slack in integration tests
firestore: increase time slack in integration tests

These tests continue to be flaky because timestamps returned
from the service are anomalously low.

Change-Id: Ibfe55ca04d632d5fd9c0cdd621ba316ddaed3738
Reviewed-on: https://code-review.googlesource.com/19450
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

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