New option to disable TopN (for testing Sort)

   When a query uses ... ORDER BY ... LIMIT ..., the planner unconditionally picks the TopN operator to (efficiently) implement this order.  This precludes an easy way to test the External Sort operator over a large dataset (e.g., test spilling).

   A new internal option to disable picking TopN (hence using the External Sort instead) would be useful for various testings. (And may be in other scenarios, like a bug found in the TopN).