Write a simplified libretro runner

[mysterymath]

We've observed emutest causing some kind of go-related segfault when interacting with the atari800 libretro core. It's quite complicated for what we actually need, and if it doesn't work across the board, it doesn't seem like a good investment in time to try to fix it up.

Instead, we should see if we can take one of the other, more stripped-down libretro frontends in pure C/C++ and implement our current test logic in it. This could become part of the SDK proper, and it would allow us to more effectively maintain and debug our test infrastructure going forward.