Even if by chance it is a mastic that has asbestos in it it won't be an issue. Asbestos fibers are only hazardous if they are very fine and get airborne. Since this is encased in the mastic material it can't become airborne so it's not hazardous. I personally would bother testing it. If you test it and it does come back at containing asbestos you will have to acknowledge that at any future transactions involving the property. If you don't test it you don't know if there's any sleeve don't have to say anything about it.