t3701: use test-terminal to collect color output

t3701: use test-terminal to collect color output

When testing whether "add -p" can generate colors, we set
color.ui to "always". This isn't a very good test, as in the
real-world a user typically has "auto" coupled with stdout
going to a terminal (and it's plausible that this could mask
a real bug in add--interactive if we depend on plumbing's
isatty check).

Let's switch to test_terminal, which gives us a more
realistic environment. This also prepare us for future
changes to the "always" color option.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>