i18n: use test_i18n{grep,cmp} in t7508

Two tests looked for "[Uu]sage" in the output, but we cannot expect the
l10n to use that phrase.  Mark them with test_i18ngrep so that in later
versions we can test truly localized versions with the same tests, not
just GETTEXT_POISON that happens to keep the original string in the
output.

Merge a few tests that were artificially split into "do" and "test output
under C_LOCALE_OUTPUT" in the original i18n patches back.

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