CBQE-1558: array_agg tests

Change-Id: Iea5c6ff87430101aa3e5cb66de5368a43e6f8910
Reviewed-on: http://review.couchbase.org/29622
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>