Maybe I'm naive, but I don't see why fully functional runtime tests are required. The only purpose of a runtime tests is to test things that could vary, at runt

Maybe I'm naive, but I don't see why fully functional runtime tests are required. The only purpose of a runtime tests is to test things that could vary, at runtime, like architectural incompatibilities, with memory layout things like endianness and bitfields being an example. These are trivial to implement in a few dozen bytes. If you have a ROM soldered into boards, then absolutely none of this would be a concern since the architecture would be intimately known, so I'm now completely lost on your point. Anyways, cheers!