If you're going to get a sample and do a strand test, maybe cut off a small piece in an area that has the most weight to it so it's not as noticeable since your hair is short. You can dye it with the henna, then, to find your answer, you can do a root retouch on it. That way, since it's not on your head, if it does have an adverse chemical reaction, there's no way you could possibly get burned or injured from it.