You could test data structures (which tend to be implemented using macros), but any nontrivial subsystem of a monolithic kernel lives in a web of dependencies with other subsystems. This is especially true of Linux.

To solve this, you would need to either use a hierarchical decomposition of subsystems or do some crazy mocking to run subsystems outside of the full kernel.

There is a recent project (KUnit) to add a unit testing framework to the Linux kernel, but it remains to be seen how much adoption it will get.