test-ordered-set: add a case where we get 0 for duplicate entries

This API is a complete mess. We forgot to do a hashed comparison for duplicate
entries and we use a direct pointer comparison. For trivial_hash_ops the result
is the same. For all other case, it's not. Fixing this properly will require
auditing all the uses of set_put() and ordered_set_put(). For now, let's just
acknowledge the breakage.