firestore: increase timestamp check slack again
firestore: increase timestamp check slack again

Timestamp comparison still failing on Travis.

Change-Id: Ic065b64062a90368392999b2a7c3db117388afc7
Reviewed-on: https://code-review.googlesource.com/17610
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