CBQE-0: add keys expr tests

Change-Id: I0f94c657d0ee8fdf7ad760fb72264fb8b04a55a7
Reviewed-on: http://review.couchbase.org/32996
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>