[test] Correctly include build llvm_shlib_dir in stand-alone builds

Add the build llvm_shlib_dir into LD_LIBRARY_PATH before the directory
specified as llvm_libs_dir, in order to fix stand-alone builds
attempting to use installed clang libraries.

In case of stand-alone builds llvm_libs_dir specifies the location of
installed LLVM libraries which can also contain an older version
(previous build) of clang libraries. Therefore, ensure to specify
llvm_shlib_dir (which is always the build tree path) before
the potentially-system llvm_libs_dir.

Differential Revision: https://reviews.llvm.org/D26115

llvm-svn: 286952