As with many things in aviation - it depends. Looking in my most recent logbook at some of the flight tests I've done over the last couple of years, the G-120TP, PC-21, P-51C, TBM900 and B-24 were all flown with instructors, so were logged as P2 . If its an SEP or LSA and the owner is just along for the ride and doesn't touch the controls I log it as P1.