The Linux kernel has not historically had great unit testing practices--it's mostly tested with large integration tests. This is in part because it inherits old development practices where unit testing was less popular, and partially because it's kind of hard to test a lot of Kernel code outside of specialized environments (e.g., would need to include a m68k emulator in the test environment to test this code).

The doubly-linked `list` type that's used ~everywhere in the kernel didn't have any in-repo unit tests until 2019 ( https://github.com/torvalds/linux/commit/ea2dd7c0875ed31955c... ), after kunit, a library for userspace testing of kernel code, was added.