CBQE-1901: add tests for unnest expression

Change-Id: If80daed204fde97c8a4523a3438329fddef87152
Reviewed-on: http://review.couchbase.org/32718
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>