bigtable/bttest: Enforce correct timestamp granularity.
bigtable/bttest: Enforce correct timestamp granularity.

Change-Id: I6c13a58be4de612196c3f5af8abe6eeee8cfcf25

googleapis/google-cloud-go,Maksadbek/gcloud-golang,blowmage/gcloud-golang,ptomasroos/gcloud-golang,Maksadbek/gcloud-golang,roth1002/gcloud-golang,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,ptomasroos/gcloud-golang,blowmage/gcloud-golang,hsinhoyeh/gcloud-golang,roth1002/gcloud-golang,hsinhoyeh/gcloud-golang