i18n: do not overuse C_LOCALE_OUTPUT

i18n: do not overuse C_LOCALE_OUTPUT

It is too coarse-grained way that led to artificial splitting of a
logically single test case into "do" and "check only without poison".
As the majority of check is done by comparing expected and actual output
stored in a file with test_cmp anyway, just introduce test_i18ncmp that
pretends the actual output matched the expected one when gettext-poison
is in effect.

Signed-off-by: Junio C Hamano <gitster@pobox.com>