tests: iterator_helpers: assert number of iterator items

When the function `expect_iterator_items` surpasses the number of
expected items, we simply break the loop. This causes us to trigger an
assert later on which has message attached, which is annoying when
trying to locate the root error cause. Instead, directly assert that the
current count is still smaller or equal to the expected count inside of
the loop.

(cherry picked from commit 9aba7636)

tests/iterator/iterator_helpers.c

@@ -51,8 +51,7 @@ void expect_iterator_items(
 				cl_assert(entry->mode != GIT_FILEMODE_TREE);
 		}
 
-		if (++count >= expected_flat)
-			break;
+		cl_assert(++count <= expected_flat);
 	}
 
 	assert_at_end(i, v);