We never launched so it was all theoretical. We never got to the point of actual radiation testing, this is done but a very advanced kind of thing to do, for example we did go to a Lockheed Martin location a few times to, test our GPS receiver which had its restrictions removed and would work in orbit. (when configured with the right firmware this had ITAR restrictions, was considered munitions, students from certain countries couldn't participate) They also had facilities for environmental testing: heat, cold, vibration, vacuum, etc. The two hardest parts of designing a satellite are attaching to the rocket and surviving launch and managing your temperature in orbit.

I don't remember all of the details of the OS level things we didn't have control of for the different OS options out there, we were just going to use Linux though, it was a viable option. One of the things you do is have a really simple hardware watchdog which will just reboot everything if it stops receiving the right health indications for exactly the case of difficult to recover faults.