I'll correct myself. That was how it was done circa 2000-2005, but my info was way out of date because I haven't followed kernel development much for a while. Here's some stuff about how it's done now. https://www.kernel.org/doc/html/latest/dev-tools/testing-ove...
Basically there is a system called KUnit for writing white box unit tests and there is code coverage to determine coverage as you would expect, then there are systems for static analysis and verifying assertions.
