MDEV-7103: Sporadic test falure in rpl.rpl_parallel_show_binlog_events_purge_logs

The test case had a classic mistake: SET DEBUG_SYNC='now SIGNAL xxx'
followed immediately by SET DEBUG_SYNC='RESET'. This makes it
possible for the waiter to miss the signal, if it does not manage
to wake up prior to the RESET.

mysql-test/suite/rpl/r/rpl_parallel_show_binlog_events_purge_logs.result

@@ -7,7 +7,6 @@ SHOW BINLOG EVENTS;
 SET DEBUG_SYNC= 'now WAIT_FOR on_show_binlog_events';
 FLUSH LOGS;
 SET DEBUG_SYNC= 'now SIGNAL end';
-SET DEBUG_SYNC= 'RESET';
 [connection slave]
 SET DEBUG_SYNC= 'RESET';
 include/rpl_end.inc

mysql-test/suite/rpl/t/rpl_parallel_show_binlog_events_purge_logs.test

@@ -22,7 +22,6 @@ SET DEBUG_SYNC= 'after_show_binlog_events SIGNAL on_show_binlog_events WAIT_FOR
 SET DEBUG_SYNC= 'now WAIT_FOR on_show_binlog_events';
 FLUSH LOGS;
 SET DEBUG_SYNC= 'now SIGNAL end';
-SET DEBUG_SYNC= 'RESET';
 
 --echo [connection slave]
 --connection slave