t6006: drop "always" color config tests

t6006: drop "always" color config tests

We test the %C() format placeholders with a variety of
color-inducing options, including "--color" and
"-c color.ui=always". In preparation for the behavior of
"always" changing, we need to do something with those
"always" tests.

We can drop ones that expect "always" to turn on color even
to a file, as that will become a synonym for "auto", which
is already tested.

For the "--no-color" test, we need to make sure that color
would otherwise be shown. To do this, we can use
test_terminal, which enables colors in the default setup.

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