[iter] Make hb_sorted_array_t work as iter

Ugly, but does the job.
diff --git a/src/test-iter.cc b/src/test-iter.cc
index 3fdf4df..2e8c408 100644
--- a/src/test-iter.cc
+++ b/src/test-iter.cc
@@ -109,6 +109,8 @@
   test_iterable (v);
   hb_set_t st;
   test_iterable (st);
+  hb_sorted_array_t<int> sa;
+  test_iterable (sa);
 
   return 0;
 }