merge tests: use "test_must_fail" instead of ad-hoc pattern

merge tests: use "test_must_fail" instead of ad-hoc pattern

As in the preceding commit change a similar fragile test pattern
introduced in b798671fa93 (merge-recursive: do not rudely die on
binary merge, 2007-08-14) to use a "test_must_fail" instead.

Before this we wouldn't distinguish normal "git merge" failures from
segfaults or abort(). Unlike the preceding commit we didn't end up
hiding any SANITIZE=leak failures in this case, but let's
correspondingly change these anyway.

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