cache.h & test-tool.h: add & use "USE_THE_INDEX_VARIABLE"

cache.h & test-tool.h: add & use "USE_THE_INDEX_VARIABLE"

In a preceding commit we fully applied the
"index-compatibility.pending.cocci" rule to "t/helper/*".

Let's now stop defining "USE_THE_INDEX_COMPATIBILITY_MACROS" in
test-tool.h itself, and instead instead define
"USE_THE_INDEX_VARIABLE" in the individual test helpers that need
it. This mirrors how we do the same thing in the "builtin/" directory.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>