In this case there does not appear to be any public evidence that they did much testing in below freezing temperatures. This isn't to say that they couldn't do

In this case there does not appear to be any public evidence that they did much testing in below freezing temperatures. This isn't to say that they couldn't do the testing, just that they didn't do it because the mission parameters did not require it. There don't seem to have been many "ongoing experiment" type sensors on the lander which would be significantly useful to keep going longer than the initial mission. As for radiation, I attended a lecture series last year about the various kinds of radiation testing involved in designing hardware for satellites, they probably did do similar things to those for satellite electronics (stuff like firing beams of IIRC electrons over the PCBs to see what parts might need reinforcement/redundancy against random charged particle strikes).