t6423: mark remaining expected failure under merge-ort as such

When we started on merge-ort, thousands of tests failed when run with
the GIT_TEST_MERGE_ALGORITHM=ort flag; with so many, it didn't make
sense to flip all their test expectations.  The ones in t6409, t6418,
and the submodule tests are being handled by an independent in-flight
topic ("Complete merge-ort implemenation...almost").  The ones in
t6423 were left out of the other series because other ongoing series
that this commit depends upon were addressing those.  Now that we only
have one remaining test failure in t6423, let's mark it as such.

This remaining test will be fixed by a future optimization series, but
since merge-recursive doesn't pass this test either, passing it is not
necessary for declaring merge-ort ready for general use.

Signed-off-by: Elijah Newren <newren@gmail.com>

t/t6423-merge-rename-directories.sh

@@ -4797,7 +4797,7 @@ test_setup_12f () {
 	)
 }
 
-test_expect_merge_algorithm failure success '12f: Trivial directory resolve, caching, all kinds of fun' '
+test_expect_merge_algorithm failure failure '12f: Trivial directory resolve, caching, all kinds of fun' '
 	test_setup_12f &&
 	(
 		cd 12f &&