CBQE-1782: add poly_length tests

Change-Id: I96b4884b9cac4a3b09365ca1126f65371450a41b
Reviewed-on: http://review.couchbase.org/31357
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>