[yjftsjthsd-h]

NetBSD has their rump kernel tech, which specifically exists to run chunks of kernel code independently. It can do a lot of neat things (I liked the "run netbsd drivers on other OSs" trick, personally), but one of the big uses they've mentioned is that it helps testing and development.

[syrrim]

A lot of the linux kernel is implemented as modules, which does allow said modules to be run under different OSes. You could probably use the module interface to implement unit testing.