Two possible ways of implementing this would be to display all devices with the Tag that you created for test by going to Menu > Systems > Tag Catalog and then selecting the name that you called the Tag and Select "Systems with tag:". This will list all device that have that particular Tag. The next step would be to select "All Rows" and then select Actions > Agent > Set Policy & Inheritance. You will then be able to assign the new test group policy from there.
The other method would be to create a query looking for devices that have the Tag you created. Then create a Server Task that has the action of "Run Query" and has Sub-Actions to Assign Policy using the special Adaptive policy that you have. Schedule the Server task to run at your desired frequency and you basically have a automated way of applying that policy to any device as long as you apply the Tag to the device.
I hope this helps.
Spot on! Many thanks for your prompt help