You can use the exportmanageddb/importmanageddb tools within the install directory of Data Store. The thing you'll need to consider, which may be tricky, is what about the services the data references in Server? You'll need to back up the Server as well. But then, you have items in the Portal that need to be created, and then other items, (such as webmaps), may have dependencies on the service items. This gets complicated rather quickly. Something you can look into is the WebGIS DR tool to take snapshots of your full deployment at a point in time, (data in the ArcGIS Data Store, any services published to any federated Server including the hosting Server, and any data, users, and content in the Portal). You can then restore the snapshot to your test site. The only caveat to this is that the tool was designed for disaster recovery, which means it's meant to either quickly repopulate a deployment with data after a disaster, or keep a standby deployment in sync with the primary. In the event that you're just repopulating a deployment or switching to your standby, your users expect to reach their services, apps, and content using the same URLs. That being said, where you take the backup has to have the same front-facing URLs as where you apply the backup. The help on disaster recovery goes into a bit of detail on this.
