Completed the fix for this bug.
Note: in 5.3 the affected 'if' statement in Item_in_subselect::single_value_transformer()
starting with the  condition (thd->variables.sql_mode & MODE_ONLY_FULL_GROUP_BY)
should be removed altogether. The change from table.cc is not needed either.
This is because in 5.3
 - min/max transformation for subqueries are done at the optimization phase
 - evaluation of the expensive subqueries is done at the execution phase.

Added an EXPLAIN EXTENDED to the test case for bug #12329653.