One thing that wonders me. The patch contains a fix but obviously no unit test that verifies it. Is that the standard of linux kernel development? In the company I work for, the patch would certainly be rejected because of that.
Here's one integration test project: https://linux-test-project.github.io/ -- typically (non-platform) subsystem maintainers write their own stress tests. But stress tests are merely probabilistic.
I'm unfamiliar with how the Linux kernel does their tests, but presumably it would require a diverse set of hardware, and a lot of code would have to be in the form of integration tests...
On most of project deliveries we have gone back to only integration testing.