[Debuginfod][unittest] Include llvm/Support/StringExtras.h

Include the "llvm/ADT/StringExtras.h" in unittest to resolve a
"use of undeclared identifier 'utostr'" error that surfaces on mac-x64
builds after https://reviews.llvm.org/D155178.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D155314