Revert r188354, "[tests] Ensure subsitution string is ascii."

It caused "shell parser error" on win32 internal shell. ShParser doesn't expect unicode, but str.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@188373 91177308-0d34-0410-b5e6-96231b3b80d8