Lowest temperature for a microcontroller
What would it be a good test scenario to setup to evaluate at that temperature the MCU still survives ?


Since you have stated in comment that the application is "safety-critical", the answer is simple - there is no good setup, as long as the results will be used to select the parts. You are using the parts outside their rating, and that is a very big no-no.

If you're determined to do it anyways, the answer is to run the part at a temperature significantly below -55 for prolonged periods, preferably for the projected lifetime of the final product. In addition, temperature cycling over the extremes expected for the final product use are also critical. It's one thing to put the box in the deep freeze for a long time, but another to repeatedly freeze and warm it - the latter is much more stressful. And "significantly" varies with test sample size. If you test a large number of parts you can get away with less temperature margin. If you're going to test just a few, you need to include margin for unit variation in the final parts.

Note that this means running the part at temperature, not just freezing it and then testing. You must check performance as you go. This is a performance test, not a storage conditions test.

"But", you whine, "that's going to take a long time and a lot of money!"

Yup. That's why mil-spec parts cost so much. And even then, those are typically qualified by running at temperature for a few hundred hours or so. However, the manufacturer can get away with such short-term production test because the development cycle did the long-term, exhaustive testing and the process and materials have enough of a track record to trust. You don't have that luxury.