tests: use test_i18n* functions to suppress false positives

tests: use test_i18n* functions to suppress false positives

The test functions test_i18ncmp and test_i18ngrep pretend success if run
under GETTEXT_POISON. By using those functions to test output which is
correctly marked as translatable, enables one to detect if the strings
newly marked for translation are from plumbing output. If they are
indeed from plumbing, the test would fail, and the string should be
unmarked, since it is not seen by users.

Thus, it is productive to not have false positives when running the test
under GETTEXT_POISON. This commit replaces normal test functions by
their i18n aware variants in use-cases know to be correctly marked for
translation, suppressing false positives.

Signed-off-by: Vasco Almeida <vascomalmeida@sapo.pt>
Signed-off-by: Junio C Hamano <gitster@pobox.com>