[tests] Change with_system_lib feature to match what XFAIL lines expect.

 - The lit builtin XFAIL handling is more restrictive than what we were
   previously using, and for now I'd rather keep the lit one restrictive.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189692 91177308-0d34-0410-b5e6-96231b3b80d8