tests: remove most uses of test_i18ncmp

tests: remove most uses of test_i18ncmp

As a follow-up to d162b25f956 (tests: remove support for
GIT_TEST_GETTEXT_POISON, 2021-01-20) remove most uses of test_i18ncmp
via a simple s/test_i18ncmp/test_cmp/g search-replacement.

I'm leaving t6300-for-each-ref.sh out due to a conflict with in-flight
changes between "master" and "seen", as well as the prerequisite
itself due to other changes between "master" and "next/seen" which add
new test_i18ncmp uses.

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