CBQE-1586: add explain tests to tuq

Change-Id: I5f291c957f3d3bb700cf8985c7aacf96aa2dc010
Reviewed-on: http://review.couchbase.org/30830
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>