config tests: simplify include cycle test

config tests: simplify include cycle test

Simplify an overly verbose test added in 9b25a0b52e ("config: add
include directive", 2012-02-06). The "expect" file was never used, and
by using .gitconfig it's not as intuitive to reproduce this manually
with "-d" as some other tests, since HOME needs to be set in the
environment.

Also remove the use of test_i18ngrep added in a769bfc74f ("config.c:
mark more strings for translation", 2018-07-21) in favor of overriding
the GIT_TEST_GETTEXT_POISON value.

Using the i18n test wrappers hasn't been needed since my
6cdccfce1e ("i18n: make GETTEXT_POISON a runtime option", 2018-11-08).
As a follow-up change to the yet-to-be-added t0017-env-helper.sh will
show, doing it this way can hide a regression when combined with
trace2's early config reading. That early config reading was added in
bce9db6de9 ("trace2: use system/global config for default trace2
settings", 2019-04-15).

So let's remove the testing for that potential regression here, I'll
instead add it explicitly to t0017-env-helper.sh in a follow-up
change.

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