MB-12251: Add tests with open ranges

The multi-dimensional index supports open ranges, either on start,
end or both sides. Add additional tests for such cases.

Change-Id: I440318d2d414cea36f5706e5f13cad150e8df8a0
Reviewed-on: http://review.couchbase.org/41770
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>