So for the test cart idea, I had initially thought something like this, maybe even Nintendo Repair Center related. There's an issue with that however and that's the discs don't do anything on retail consoles. It says they can't be read. I've tried on 3 different Wiis (including a modded one), a Wii U kiosk unit and retail unit, dev unit for Wii and Wii U, etc. If it was meant to provide a video test signal I would think it should work on retail consoles with no other equipment needed but they don't.
As for the second part, I think it's very likely it has something to do with the Gamecube. I'm not sure it would be Gamecube Firmware for the Wii, however. Like I mentioned, the disc was written back in 1999. That's a long time to hold onto a disc of testing files that were made VERY early in the Gamecube development. I would think if they wanted to test Gamecube firmware in the Wii, it would be on a disc from that time frame especially because the console would have gone through radical changes since initial development and the files on a 1999 disc would've been outdated/incompatible. I mean the Dolphin units themselves used carts for development at first, then they switched it over to discs about halfway through development life.
This is why I'm so lost on these discs. They don't seem to have a straightforward answer.