Of course, 99999 out of 100000 tests are going to pass every time. But processing power is so cheap, why not let it work for you? You never know what might have broken if there's no test.
And, once you have found an unexpected regression, you can then write a test for it if there wasn't one before. Having that test available, as part of a loadable "test" module, say, in a generic kernel release then means it can be triggered in the field on request by anyone at all with the latest update and whatever oddball hardware they have. This would greatly increase the result data and illuminate the circumstances and manner in which the test passes or fails.
And there's nothing like having to write unit tests to thrash out the idiocies and dark corner cases of a new interface. You don't even have to run them initially, the mere mental questioning debugs the design before its stupidities get coagulated into something you're going to have to maintain for years afterward.
I have myself grumbled about having to write tests. But I have never regretted the payoff in quality. And it's very satisfying to see the new release of one's library run its test suite in the blink of an eye and know that you didn't break anything important with your last changes. Or maybe you did, and you get to save yourself a wasted release and maybe a brown paper bag too.
And of course, the test results are gold dust to anyone who wants to document the interfaces. I hadn't heard that the Linux kernel's documentation has been a howling success story. A more formal requirement to write unit tests as part of the kernel development process would go far to improving things. And, dammit, it's just satisfying to know that what you wrote definitely works.
I fully accept that there is no getting around the need for skill and interaction in tracking down the more devious regressions. It's just that we should work towards an environment where that is made as easy as possible.
I hope that new kernel regression trackers are soon appointed and get the support and remuneration their important job deserves. If not, one is saying in the loudest possible language that, words aside, quality is actually only for wimps and doesn't really matter.