Testing is answering "does it do, what it is supposed to do?" and autonomous means "according to it's own law(s)". Sounds like a contradiction to me. I'd answer with "none".

We did not fail to automate testing. We automated the easy parts and kept calling it progress. The hard part was never running tests. It was deciding what actually matters. We cannot automatically detect risk. Risk lives in context, trade-offs, and user expectations...none of those are deterministic.

Historically speaking, if an autonomous system found a bug [monkeys and fuzzers have done that for years ;)], you'd just get a "test failed" log. You'd still need to manually dig through the stack traces to pinpoint the "why" of it. Well, an Ai app can tell you that the system crashed or a button moved 5px to the left. But it still can't tell you if that movement broke the UX without a human explicitely defining the heuristic first.

IMO, the hard parts that actually getting solved right now is the autonomous serialization of state; context capture and issue repro steps to be precise (and definitely not the "finding of bugs" part).