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

As in the preceding commit change a similar fragile test pattern
introduced in b798671fa9 (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>

t/t6407-merge-binary.sh

@@ -43,14 +43,9 @@ test_expect_success resolve '
 	rm -f a* m* &&
 	git reset --hard anchor &&
 
-	if git merge -s resolve main
-	then
-		echo Oops, should not have succeeded
-		false
-	else
-		git ls-files -s >current &&
-		test_cmp expect current
-	fi
+	test_must_fail git merge -s resolve main &&
+	git ls-files -s >current &&
+	test_cmp expect current
 '
 
 test_expect_success recursive '
@@ -58,14 +53,9 @@ test_expect_success recursive '
 	rm -f a* m* &&
 	git reset --hard anchor &&
 
-	if git merge -s recursive main
-	then
-		echo Oops, should not have succeeded
-		false
-	else
-		git ls-files -s >current &&
-		test_cmp expect current
-	fi
+	test_must_fail git merge -s recursive main &&
+	git ls-files -s >current &&
+	test_cmp expect current
 '
 
 test_done