I agree with everything you said, but I even think it's unfair to say they're "testing Newton's laws." Rather, I think they're often testing "is it feasible for us to reproduce the idealized circumstances from thought experiments." Case in point is firing an object in the opposite direction of a moving vehicle. I don't think any of them questioned the physics behind it, but it was really a question of if they could contrive the circumstances exactly so they observed the kind of result one would see without all of the nasty effects that reality imparts (like spin, air resistance, speed variance, etc.): http://www.youtube.com/watch?v=BLuI118nhzc