test-lib: extend test_decode_color to handle more color codes

Enhance the test_decode_color function to handle all common color codes,
including background colors and escapes that contain multiple codes.
This change necessitates changing <WHITE> to <BOLD>, so update t4034
as well.

This change is necessary for the next commit in order to test
background colors properly.

Signed-off-by: Kevin Ballard <kevin@sb.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>