rev-list tests: don't hide abort() in "test_expect_failure"

rev-list tests: don't hide abort() in "test_expect_failure"

Change a couple of uses of "test_expect_failure" to use a
"test_expect_success" to positively assert the current behavior, and
replace the intent of "test_expect_failure" with a "TODO" comment int
the description.

As noted in [1] the "test_expect_failure" feature is overly eager to
accept any failure as OK, and thus by design hides segfaults, abort()
etc. Because of that I didn't notice in dd9cede9136 (leak tests: mark
some rev-list tests as passing with SANITIZE=leak, 2021-10-31) that
this test leaks memory under SANITIZE=leak.

I have some larger local changes to add a better
"test_expect_failure", which would work just like
"test_expect_success", but would allow us say "test_todo" here (and
"success" would emit a "not ok [...] # TODO", not "ok [...]".

So even though using "test_expect_success" here comes with its own
problems[2], let's use it as a narrow change to fix the problem at
hand here and stop conflating the current "success" with actual
SANITIZE=leak failures.

1. https://lore.kernel.org/git/87tuhmk19c.fsf@evledraar.gmail.com/
2. https://lore.kernel.org/git/xmqq4k9kj15p.fsf@gitster.g/

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