[test] Use printf instead of C++ iostream, NFC.

This test fails with libc++ when built with MemorySanitizer. This
is because we link to an uninstrumented version of the library
so msan detects a nested error when calling std::cout << "...".
This can be easily avoided by using good old printf.

Differential Revision: https://reviews.llvm.org/D49867

llvm-svn: 338053