I HAVE heard about these carts... apparently they were used to test NES consoles and accessories in Nintendo repair shops back in the day, or something like that. From what I've seen in my research, there's a variety of those test carts (Duck Hunt, ROB, Power Pad, Port Test, and a few others), but each of them is fairly rare (some only have one known copy, others are a little bit more common like Duck Hunt and Zelda).
From what I've heard, at least in Zelda's case, it's no different from the actual cartridge other than the yellow casing. Finding actual concrete information on those isn't easy (or maybe I'm just being stupid in how I'm searching for it).