Actually, a low TTL is ideal for testing purposes. While you're correct that a query will never give a different answer, a low TTL ensures that the name won't linger in any resolver's cache for very long, which makes it less easy to discover. This also makes the arbitrary string chosen as a subdomain particularly ephemeral, which is important when testing name-based virtual hosts. Why leave a testing domain stuck indefinitely in the cache of a resolver I don't control? I'd rather have it disappear when I'm not using it.