Fix iterator expiry test

This test started failing with Erlang 20.0 release. The reason is that opaaque
NIF resources stopped being identifed as empty binaries in Erlang so previously
it matched but once refs were used it stopped matching.

Fixes #855
diff --git a/test/khash_test.erl b/test/khash_test.erl
index 20bd785..f19c858 100644
--- a/test/khash_test.erl
+++ b/test/khash_test.erl
@@ -240,7 +240,7 @@
                     }
                 end,
                 fun({ok, H, I, Err}) ->
-                    {ok, I} = khash:iter(H),
+                    {ok, _OtherI} = khash:iter(H),
                     {foo, bar} = khash:iter_next(I),
                     end_of_table = khash:iter_next(I),
                     {