I agree that tests will not be worthwhile, no single test will be useful across species (or sub-species in the case of dogs). But intelligence can be roughly compared between animals based on observations of complex behavior. We can see that dogs have a very high level ability to communicate and interact not just with humans but other animals as well. Different dog breeds show different abilities in problem solving, and perhaps on average across the breeds not at the level of wolves, but some dogs do show initiative in problem solving without human prompting.
We can’t know for sure if house cats are silently planning global domination, or if dogs are working on a quantum theory of gravity instead of chasing rabbits when they’re legs twitch while they sleep, but we can observe their behavior and make some reasonable though rough assumptions about the level of intelligence needed.
BTW: Yes, cats are planning world domination and they would have gotten away with it by now if it wasn’t for those meddling dogs.