Saying Brook is a good test before Canelo cause his speed is like saying Chavez Jr. is a good test before Golovkin cause of his size. Oversimplification and not true.
and just because Golovkin's recent fight was against a tougher opponent than Canelo's we gonna forget about how much better opponents Canelo faced otherwise?
If we would have to rank all their opponents, out of the top 5, 4 would be Canelo opponents (Mayweather, Lara, Trout, Cotto) and only one Golovkin opponent (Jacobs)
Also the Jacobs case...
Did Jacobs prove that he is better than we thought? Yep. But you gotta remember that the guy was a 15-1 underdog and he ended up beating Golovkin in many people's eyes. Now it's not fair from Golovkin fans to claim that the guy is some extra class if they originally thought he will be a food for Golovkin.
Canelo struggled against opponents who he supposed to struggle against. Nobody thought before he fought Lara or Trout that he's gonna wash those guys. In fact they were very difficult style matchups for Canelo more difficult on paper than Jacobs to Golovkin.
Golovkin is all about forward momentum and his preferred range. Take away from him these and you win. Not many fighter can do it and it's easier said than done but Canelo is able to do it imo... He gonna back Golovkin off and imo maybe bully him at points. Golovkin moves forward but when he get inside he rather tries to reset and try again than rough his opponent up so if somebody can stand his ground without absorbing much damage or move laterally consistently he should be able to beat Golovkin.
For the former one needs good defense, good reflexes to slip punches, an arsenal to land punches on Golovkin and at least respectable power to back him off. Sound like Canelo to me.