[llvm] [ci] Handle the case where all reported tests pass but the build is still a failure (PR #120264)

[Lucile Rose Nihlen via llvm-commits]

https://github.com/lnihlen commented:

Thanks for working this up. I'd like @boomanaiden154 to verify this keeps the new infra happy too, and had a question about fragility in one of the tests, but otherwise LGTM

https://github.com/llvm/llvm-project/pull/120264