COSTESTIMATIONTEST.MERGEMANYBUFF

The memory leak was due to the constructor for the 
Fake_Cost_model_server class (stub implementation for
the real Cost_model_server class) allocated a
Server_cost_constants object that was not deleted.

To ensure the Server_cost_constants object is deleted,
this patch adds the following:
-a destructor for the Fake_Cost_model_server class that
 deletes the allocated Server_cost_constants object
-makes the destructor for the Cost_model_server class
 virtual to ensure that the correct destructor is called.