spanner: skip TestIntegration_StartBackupOperation
spanner: skip TestIntegration_StartBackupOperation

Updates #2393

Change-Id: I9edfa9c214f4448d2bf12a01743e8640674f8d6d
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/57151
Reviewed-by: Cody Oss <f11fa1e55ff5638510f5bf86ea76672a630250f9@google.com>

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