CBQE-1531: add group by clause tests

Change-Id: I6b373a75b7cda6540da2ad7d090cc643dd6cbf13
Reviewed-on: http://review.couchbase.org/29206
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>