What Mel said ... PLUS ...
The "purpose" of all flights during phase I is flight testing the aircraft. Therefore, flight instruction cannot be the "purpose of the flight."
I know some people will agree with your theory. But this is discussed every year at our DAR seminars and it is simply not acceptable.
Your question was directed to Mel, but I, like others have done test flying of friends' airplanes. Below are some thoughts that fortunately, I did not have to share with them explicitly.
1. Test flying is NOT a time for distractions .... you would be a distraction.
2. Training in an UNTESTED plane is NOT the place for training... you would be in one.
3. UNTESTED planes with pilots-to-be undergoing training are most likely NOT insured ... that may not matter to you, but you would be under said conditions.
4. If you ignored all of this and went ahead anyway and got caught, it would most likely make things just a bit more burdensome for all the rest of us, so PLEASE do NOT do that. The rules that are in place give us a lot of leeway. We do not need to stretch them as if we do, they may snap back and overly constrain us all.
It seems that you have stayed safe and wise all this time. Another month or so to do it RIGHT "won't kill you".
When is has been thoroughly tested, you will have a plane that you will be able to comfortably enjoy whether you are just in the pattern getting some training or taking cross country with a good friend.