Screw stuck on backflow test cock - thoughts before replacing?
You need a right angled screw driver with a long handle for applying maximum force to the valve.
Here's a DIY version: attach a locking pliers (vise grip) to the handle of a screw driver. You should now be able to press the slot-head forcefully into the valve slot while wrenching it counterclockwise (loose).
If you can shut the water off before the back flow valve you could than remove the test port and work on it more comfortably (in a vice).