unwritable tests: assert exact error output

In preparation for fixing a regression where we started emitting some
of these error messages twice, let's assert what the output from "git
commit" and friends is now in the case of permission errors.

As noted in [1] using test_expect_failure to mark up a TODO test has
some unexpected edge cases, e.g. we don't want to break --run=3 by
skipping the "test_lazy_prereq" here. This pattern allows us to test
just the test_cmp (and the "cat", which shouldn't fail) with the added
"test_expect_failure", we'll flip that to a "test_expect_success" in
the next commit.

1. https://lore.kernel.org/git/87tuhmk19c.fsf@evledraar.gmail.com/T/#u

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

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