After a bunch of rumours way back when, it seems the truth has emerged and now with video proof, Zelda testing levels remain intact on The Wind Waker.
"Coders discovered ActionReplay codes that unlock trippy MonkeyBall-looking worlds that look completely like any Zelda experience you’ve ever played. Cellda’s Link finds himself in an Akira circus nightmare with checkered placeholder textures, long lakes, blocky bluffs, and a labyrynth of ladders right out of American Gladiators."
Obviously you can't get to these levels via the normal gameplay interface, but using an Action Reply cheat system, gamers are able to unlock these un-seen areas.
Basically these areas would of been used by the developers whilst they were testing the game such checking the camera angles, testing for gameplay glitches. If you already have an Action Reply you can find all the codes over at Destructoid.com.