Sporadic test falure in rpl.rpl_parallel_show_binlog_events_purge_logs

The test case rpl.rpl_parallel_show_binlog_events_purge_logs fails
sporadically.

The problem is this in the test case:

SET DEBUG_SYNC= 'now SIGNAL end';

SET DEBUG_SYNC= 'RESET';

This pattern is always wrong. The RESET will race with whoever is waiting for
the previous SIGNAL. If the waiter loses the race, it will never wake up, as
the signal is cleared before the waiter sees it.