Correctly set rpath for unittests.

This lets us run the unittest from the command line without setting
LD_LIBRARY_PATH.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202509 91177308-0d34-0410-b5e6-96231b3b80d8
With rpaths being set correctly, SHLIBPATH_VAR is not needed anymore.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202510 91177308-0d34-0410-b5e6-96231b3b80d8