The implementation of the walk method for the class Item_in_subselect
was missing. As a result the method never traversed the left operand
of any IN subquery predicate.

Item_exists_subselect::exists2in_processor() that performs the
Exist-To-In transformation calls the walk method to collect info
on outer references. As the walk method did not traverse the
left operands of the IN subqueries the outer references there
were not taken into account and some subqueries that were actually
correlated were marked as uncorrelated. It could lead to an
attempt of the materialization of such a subquery.

Also added a cleanup for some test cases merged from 5.5.