removed influence of some tests on other tests


mysql-test/r/multi_update.result:
  removed influence on other tests
mysql-test/r/subselect.result:
  if exists should not be used inside test
mysql-test/t/multi_update.test:
  removed influence on other tests
mysql-test/t/subselect.test:
  if exists should not be used inside test
mysql-test/t/system_mysql_db_fix.test:
  removed influence on other tests