Nuclear proliferation is a threat to all of mankind. Anything we can do to reduce the number of nuclear weapons will increase safety for all mankind. I could see how it's unfair for some countries to have nuclear weapons while others don't, however, that is the way the cookie crumbles.
No, nuclear test ban treaties are not a good idea, because a law only prevents the lawful from doing a certain behavior. Countries who are intent on hurting Americans will do so, whether there is a test ban treaty or not. Countries need to be able to defend themselves, because some nations will not be happy until the United States no longer exists.