CBQE-2486: add tests for EXISTS

Change-Id: I7a9f33aa6928af56c3c66753f9698b183b94a7a5
Reviewed-on: http://review.couchbase.org/43684
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: buildbot <build@couchbase.com>