Riding an unregistered and uninsured motorcycle?

Ok so, my actual question isn't as stupid as the title makes it sound. I want to buy a motorcycle from a private seller, but there are no plates on it or insurance. I currently have my permit, and the laws in NY say I have to be within 1/4 mile of a licensed rider over 21. If I were to take the bike out on a test ride, with a licensed rider staying within 1/4 of me, will a cop really care? I mean, I can't just buy the bike without knowing how it rides on a real road (Not just a residential road where I can't go more than 15mph). If it makes any difference, the other rider with me will be a friend who is an Army recruiter, and will be coming from work so he'll probably in uniform. I'm going to buy the bike as long as everything goes well so I was thinking I could get insurance for it anyways. Would this make a cop less angry if I were to get pulled over? And just for the record, I plan on riding it completely normally; no lane splitting, speeding, etc etc. The only thing the cop would pull me over for is not having a plate. Isn't there something like a 10 day grace period to get a bike registered and insured after the date of purchase? How do people test ride bikes from dealerships?