Few days ago I wrote I have a world's fastest broken SS-5. The problem is that it is so fast that this alone makes it broken.
It looks like at least some PromDiag/POST/OBP tests fail just because qemu doesn't emulate cpu cycle-exact. It can be they wait that an irq would happen while they execute like 100 nops, but qemu nop is much faster than a real one, so an irq comes too late. "nop" is just an example here, I didn't disassemble the tests yet, but it looks very much like it: the timer test passes if I make the timer tick 256 times faster.
Probably the other tests fail due to the same reason. So the OBP timer/irq tests are probably useless.