Instructions on test DLC

Steps to re-generate the test DLC payload and its artifacts

./generate_test-dlc_payload.sh

Extra notes:

• dlcservice_test-dlc.payload is used by tast test platform.DLCService and is unlikely updated unless we see a test regression.

• When a test regression happens and we decide updating a new payload is the right cure, then platform.DLCService needs to be updated to point to the new payload location (if changed) or payload DLC ID (if changed).