Let test/Unit/lit.cfg add config.shlibdir to $PATH on DLL platforms like cygming.

This makes unittests run with BUILD_SHARED_LIBS on DLL platforms.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212316 91177308-0d34-0410-b5e6-96231b3b80d8