Would probably start off from customer base then. It takes time to test and report issues, so they would probably have to think to some incentive. If customer base it too low, then the question is rather how to attract new customers...
I would suggest that they develop and use the tools themselves. There won't be any better and diligent testers. Once customers join in they would have zillion of requests/ideas and then they have to sort between all of them to keep the product sound.
With plain logic application, writing good unit tests is much harder than writing good code.
Probably by a magnitude of 10 or more. Think about it, you have to surround every path in code by a test case !
You have to create mocks everywhere and end up consuming pretty much the whole team time in writing and maintaining tests, whereas in less time through code reviews/rewrites you could have fixed several issues, improved readability, improved team knowledge, facilitate long term support, and so on...
In software real troubles comes from integration, not much unitary stuff (unless you praise Monkey coding).
And for Linux, it is not just a plain logic application but a kernel that runs on different hardware and has huge base of users and applications. Testing the thing as whole is the only thing that matters. Releasing alpha and beta software is a much more sound, rational and efficient approach. IT industry is (or at least was) organized to test before production.
I am certainly not against unit testing. It remains a wise approach for piece of software that need to be glued in stone forever or have super bounded in/out outcome (eg: bank transfers). But it is nothing more than a tool that you might use or not based on situations. Certainly never a one fit all solution !