Scientist is to the Test Subject as Observer is to...
This is something I had to ponder for a while. I do agree with the earlier statement of Scientists also behaving as Observers, but that Observers do not behave as Scientists.
I would like to expand on and postulate possible answers from this.
The first thing one must ask himself in this dilemma is, what do Observers observe? People, things, reactions, interactions, pretty much anything that can be looked at. Perhaps a word you might be looking for is Specimen.
Maybe not. Let us dig a bit farther, shall we?
Beyond just the physical of observing people walking by or a bag blowing across the street, two dogs playing with each other, etc. Observers take note of things beyond our physical realm. Emotions, desires, speech patterns.
Observers observe the abstract. They observe what other people do not, but that is getting beyond the point of your question.
There are more definitions beyond what has been talked about:
observing a holiday, observing a moment of silence for the dead, observing a law.
In order to meet most (if not all) definitions of the word, I can think only of the following:
Scientist is to Test Subject as Observer is to things, people, or situations that deserve to be recognized.
As for a shorter version of this, I have drawn a blank.
I do hope this helps on your endeavor; it certainly was fun to think about and delve into.