CBQE-0: n1ql: add index tests

Change-Id: Ida6c8c83bea88f48a49938311cea67392293503e
Reviewed-on: http://review.couchbase.org/42420
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>