test-tool.h: include git-compat-util.h

test-tool.h: include git-compat-util.h

The test-tool programs include "test-tool.h" as their first
include, which breaks our CodingGuideline of "the first
include must be git-compat-util.h or an equivalent".

Rather than change them all, let's instead make test-tool.h
one of those equivalents, just like we do for builtin.h
(which many of the actual git builtins include first).

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