I partially agree. You are right, you can not contrive a test for every situation. But, I'd bet that you can to exceed the 150,000 mile rate... Here's how.
Now this is going out on a limb as I know more than the average bear about software and neural nets, but just enough to be dangerous about vehicle autonomy.
At the end of the day, it's all software, some procedural code, some NN, and all of it fed a stream of input data that controls servos. Devs already write tests for all of the procedural stuff daily. That's easy... But, I've recently read that the engineers at Wamo are now creating virtual environments used to train their NN systems so they can time-compress the training process...
Here is the limb... I bet that they have also devised their own internal test jigs for these virtual environments too. My prediction is that this training tool evolves into a test platform, one where real world scenario data, say, 150,001 miles worth, or billions in Tesla's case, would be captured and fed into these simulation jigs, and then the various manufacture computers and software could consume this artificial environment and be scored on how well they do.
Just a theory...