Hi
You are correct.
In Test Mode the market is settled only from the result downloaded from Betfair at the end of the game. This is how Test Mode works. In the Next Goal market the result at the end of the game will always be "No Goal", so you cannot use test mode for this market for settling purposes.
What you can do in your trigger is detect when a goal has been scored using market_score, and for example you can set stake to double, or set a user variable, or activate another trigger etc.