t: add matching negative attributes to test_decode_color

t: add matching negative attributes to test_decode_color

Most of the ANSI color attributes have an "off" variant. We don't use
these yet in our test suite, so we never bothered to decode them. Add
the ones that match the attributes we encode so we can make use of them.

There are even more attributes not covered on the positive side, so this
is meant to be useful but not all-inclusive.

Note that "nobold" and "nodim" are the same code, so I've decoded this
as "normal intensity".

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