What's another word for Guinea Pig, i.e. when you call someone a "test dummy"?

If they are participating in a scientific research study, an individual would be called a participant and a group would be called a test group.

If the group is surveyed without an actual manipulation, it would be described as a focus group. The individuals would still be called participants.

Since you've edited your question, it seems you want John's friend to use a word that implies that John is just a pawn. Rather than something that is specifically about research participation, I'd recommend one of my favorites, patsy, which is insulting and implies that John is mindlessly and foolishly going along with whatever he is told to do.

If you want a phrase that is patently offensive, you could use meat puppet.

You can use test subject or just subject

subject : 6. a person who is subjected to experimental or other observational procedures; someone who is an object of investigation; "the subjects for this investigation were selected randomly"

If "lab rat","test bunny" or "weather balloon" have fallen into overuse ,I suggest "mineshaft canary" as an option worthy of consideration.