cloud/storage: improve storage integration tests
cloud/storage: improve storage integration tests

- Invoke multiple Writes to see if the upstreaming connection
is opened once.
- Test zero content objects.

Change-Id: Ibcb47b078269c7926b01202bf2128b74eda2f9bd

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