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

[Raul Tambre via llvm-commits]

tambry wrote:

@arsenm Correct. I had another look now and managed to simplify this further. We can actually put the plugin into lib and just rely on the RPATH for it to be found. It's marked `BUILDTREE_ONLY` so it won't get installed and there seem to be other tests that rely on this too. I think `PluginsTest.cpp`, which the existing code was likely copy-paste from, could be simplified similarly, but I'll leave that as a follow-up.

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