Problem: the test waits for a 'DROP TEMPORARY TABLE' event to
appear in the master's binlog, then checks on the slave whether
the number of temporary tables has decreased. The slave does
not sync, causing a race.
Fix: check for the 'DROP TEMPORARY TABLE' event on slave
instead of on master.


mysql-test/suite/rpl/t/rpl_trunc_temp.test:
  - Fixed BUG#37493 by waiting for the event on the slave
  instead of on the master.
  - Added comments.