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 <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>