CBQE-1531: add tests for aggregate fns for query tool

Change-Id: Iad2b233e9544e9c12fe7f35f1256849996d0e65d
Reviewed-on: http://review.couchbase.org/29242
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>