I would be shocked if they weren't using a test suite, especially given all the platforms and devices Linux supports. POSIX has a test suite, and there are several Linux test suites [1], [2]. Although, I would think that an architecture port is fairly straightforward. It's reverse-engineering and writing all the device drivers, but devices generally have a well-known interface (and, therefore, presumably tests). The OpenGL drivers are being tested against the official OpenGL test suite.
Of course, there are no guarantees that it runs correctly. Probably doesn't, given that even Apple and Microsoft's software don't run correctly, either. But saying software doesn't run perfectly in all cases is almost tautological.
Of course, there are no guarantees that it runs correctly. Probably doesn't, given that even Apple and Microsoft's software don't run correctly, either. But saying software doesn't run perfectly in all cases is almost tautological.
[1] https://github.com/phoronix-test-suite/phoronix-test-suite
[2] https://github.com/linux-test-project/ltp