test-lib: use $1, not $@ in test_known_broken_{ok,failure}_

test-lib: use $1, not $@ in test_known_broken_{ok,failure}_

Clarify that these two functions never take N arguments, they'll only
ever receive one. They've needlessly used $@ over $1 since
41ac414ea2b (Sane use of test_expect_failure, 2008-02-01).

In the future we might want to pass the test source to these, but now
that's not the case. This preparatory change helps to clarify a
follow-up change.

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