[llvm] [llvm][test][NFCI] Use absolute build path instead of a relative path (PR #159126)

[James Henderson via llvm-commits]

jh7370 wrote:

I don't think it's a good idea for me to review this. I've added some other reviewers, who reviewed the original PR that introduced the PR. Also pinging @Tcc100, since they were the one to originally introduce the test.

https://github.com/llvm/llvm-project/pull/159126