t1007: modernize malformed object tests

t1007: modernize malformed object tests

The tests in t1007 for detecting malformed objects have two
anachronisms:

 - they use "sha1" instead of "oid" in variable names, even though the
   script as a whole has been adapted to handle sha256

 - they use test_i18ngrep, which is no longer necessary

Since we'll be adding a new similar test, let's clean these up so they
are all consistently using the modern style.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>