It's amazing how much one motivated person can achieve. Also, how competent the result seems to be. The test suite alone is pretty impressive.

c-testsuite is not made by the author of Kefir.

c-testsuite itself was curated from simple-cc and tinycc's test files, the latter originated from picoc.

c-testsuite is amazing. i use it to test my compiler, but even after passing the test. my compiler still choke on some real world project. also the tests lack many of arcane preprocessor rules. so you still need to read https://www.spinellis.gr/blog/20060626/ in order to understand hideset and its macro expansion algorithm.