mktag tests: improve verify_object() test coverage

mktag tests: improve verify_object() test coverage

The verify_object() function in "mktag.c" is tasked with ensuring that
our tag refers to a valid object.

The existing test for this might fail because it was also testing that
"type taggg" didn't refer to a valid object type (it should be "type
tag"), or because we referred to a valid object but got the type
wrong.

Let's split these tests up, so we're testing all combinations of a
non-existing object and in invalid/wrong "type" lines.

We need to provide GIT_TEST_GETTEXT_POISON=false here because the
"invalid object type" error is emitted by
parse_loose_header_extended(), which has that message already marked
for translation. Another option would be to use test_i18ngrep, but I
prefer always running the test, not skipping it under gettext poison
testing.

I'm not testing this in combination with "git replace". That'll be
done in a subsequent commit.

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