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

[Matt Arsenault via llvm-commits]

arsenm wrote:

I suppose this is only an issue because the plugin library is built into unittests, and not lib? The other sample plugin binaries just go to the ordinary lib directory 

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