MDEV-18450 followup: fixing rpl_shutdown_wait. The test

appeared to leave warning due to expectable full exit of IO threads.
Fixed to ignore warnings at the final START SLAVEs.

mysql-test/suite/rpl/include/rpl_shutdown_wait_slaves.inc

@@ -76,8 +76,16 @@ EOF
 DROP TABLE t1;
 
 --connection server_2
+--disable_warnings
 --source include/start_slave.inc
+--enable_warnings
+
 --connection server_3
+--disable_warnings
 --source include/start_slave.inc
+--enable_warnings
+
 --connection server_4
+--disable_warnings
 --source include/start_slave.inc
+--enable_warnings

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

@@ -35,16 +35,10 @@ connection server_1;
 DROP TABLE t1;
 connection server_2;
 include/start_slave.inc
-Warnings:
-Note	1254	Slave is already running
 connection server_3;
 include/start_slave.inc
-Warnings:
-Note	1254	Slave is already running
 connection server_4;
 include/start_slave.inc
-Warnings:
-Note	1254	Slave is already running
 connection server_2;
 include/stop_slave.inc
 include/start_slave.inc

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

@@ -22,14 +22,8 @@ connection server_1;
 DROP TABLE t1;
 connection server_2;
 include/start_slave.inc
-Warnings:
-Note	1254	Slave is already running
 connection server_3;
 include/start_slave.inc
-Warnings:
-Note	1254	Slave is already running
 connection server_4;
 include/start_slave.inc
-Warnings:
-Note	1254	Slave is already running
 include/rpl_end.inc