[XRay] Add LD_LIBRARY_PATH to env variables for Unit Tests

Summary:
This change allows us to use the library path from which the LLVM
libraries are installed, in case the LLVM installation generates shared
libraries.

This should address llvm.org/PR39070.

Reviewers: mboerger, eizan

Subscribers: mgorny, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D52597

llvm-svn: 343280