I just want to know if Unity supports for Wii U, and if it does, how do you use the controls and how do you test it on the Wii U?
Yes it works. if you want to go this route, you'll need a DevKit from Nintendo, first you'll have to sign-up as a licensed dev: https://developer.nintendo.com/
Actually maybe I could, but my main concern is testing the game. I don't know how to hook up a gamepad to a computer, or there is maybe another way.
No there is not. If you want to test your game on the actual device with correct specs, you'll need that TestKit/DevKit. If you just want to get the Input with the Tablet, I might suggest you'll look into the Rewired-Plugin in the AssetStore, it doesn't support the Tablet-Controller out of the box, but you can write your own Custom Controller in order get it to work.