[libc++/abi] Clean up uses of <iostream> in the test suite

We used <iostream> in several places where we don't actually need the
full power of <iostream>, and where using basic `std::printf` is enough.
This is better, since `std::printf` can be supported on systems that don't
have a notion of locales, while <iostream> can't.

GitOrigin-RevId: cc69d211d0d65d7bf0335fecbc323f784ac3afcc
11 files changed