Added a test case for bug #9516.
item_subselect.h:
  Fixed bug #9516.
  The bug was due to that fact that the class Item_subselect
  inherited the generic implementation of the function
  not_null_tables that was not valid for the objects
  of this class. As a result evaluation of the
  not_null_tables attribute was not correct for subqueries.
  This caused invalid transformations of outer joins into
  inner joins.