t7508: use test_terminal for color output

t7508: use test_terminal for color output

This script tests the output of status with various formats
when color is enabled. It uses the "always" setting so that
the output is valid even though we capture it in a file.
Using test_terminal gives us a more realistic environment,
and prepares us for the behavior of "always" changing.

Arguably we are testing less than before, since "auto" is
already the default, and we can no longer tell if the config
is actually doing anything.

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