A project as large and well funded as the Linux kernel could have a hardware test farm at least with reasonable coverage of popular hardware. "But Linux isn't that well funded!" Sure, but it's orders of magnitude better funded than Embassy[0], which runs tests on real hardware automatically before every merge.

There's also the Linux testing project, which is technically third party. It's not clear to me how extensive it is but for a project as important as Linux I think it has to be graded as "needs improvement."

[0] https://github.com/embassy-rs/embassy

[1] https://linux-test-project.github.io/