fixed test func_gconcat to be repeatable independent of presend tables and avoid removing user tables


mysql-test/r/func_gconcat.result:
  removed warning in test
  fixed table tnames (should be tN, where N is number)
mysql-test/t/func_gconcat.test:
  removed warning in test
  fixed table tnames (should be tN, where N is number)
sql/item_sum.cc:
  added debug information
  fixed memory leak in group_concat function