Problem: master executed a statement that would fail on slave
(namely, DROP USER 'create_rout_db'@'localhost').
Then the test did:
  --let $rpl_only_running_threads= 1
  --source include/rpl_reset.inc
rpl_reset.inc calls rpl_sync.inc, which first checks which of
the threads are running and then syncs those threads that are
running. If the SQL thread fails after the check, the sync will
fail. So there was a race in the test and it failed on some
slow hosts.
Fix: Don't replicate the failing statement.