benchmark

Issue1731 created console does not receive output (#1732)

* Instead of directly comparing std::cout and GetOutputStream(), the underlying buffers are retreived via rdbuf(), and then compared.

* Instead of fflush(stdout), call out.flush().
Use out << FormatString() instead of vprintf

---------

Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>