What the hell is this.. unfortunately, i did text back to "stop"! I also called back the number and left a message, telling them that to remove my number off their calling/text list, didn't leave my number, but i'm pretty sure they caller ID. I'm currently searching the the internet to report this. My number is on the "do not call" list, and this applies to texts too!
File a complaint here. Takes about 2 mims and can be submitted online. The more complaints they get about 347 spam texts, the better it will be. It's a hassle I know, but if you answered the test with STOP, ( as I did ) then I'm afraid you've just set yourself up to get more spam.