The thesis should be your statement about which side you support. If you disagree with animal testing, say something like "Animal testing has a negative effect on animals that negates the benefits". If you agree with animal testing, say something like, "Animal testing is a beneficial way to assure the safety of our products." The thesis should only be about one-two sentences. If you wanted to, you could probably use the first two sentences as part of your introduction.
I hope I helped!