tests: test terminal output to both stdout and stderr

tests: test terminal output to both stdout and stderr

Some outputs (like the pager) care whether stdout is a
terminal. Others (like progress meters) care about stderr.

This patch sets up both. Technically speaking, we could go
further and set up just one (because either the other goes
to a terminal, or because our tests are only interested in
one). This patch does both to keep the interface to
lib-terminal simple.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>