MDEV-30460 rpl.rpl_start_alter_restart_slave sometimes fails in BB with result length mismatch

The test was used to fail because of lacking a synchronization point
designating an expected for processing "CA_1" event has been indeed
taken into this phase.
The event apparently may not have even arrived at slave.

Fixed with deploying the missed synchronization.

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

@@ -52,6 +52,7 @@ master-bin.000001	#	Query	1	#	use `test`; create table t3( a int primary key, b
 # let's stop at first CA processing (in process_commit_alter)
 connection slave;
 include/sync_with_master_gtid.inc
+# wait for CA_1 waiting itself
 connect extra_slave,127.0.0.1,root,,test,$SLAVE_MYPORT;
 stop slave;;
 connection slave;

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

@@ -73,6 +73,10 @@ create table t3( a int primary key, b int) engine=innodb;
 
 --connection slave
 --source include/sync_with_master_gtid.inc
+--echo # wait for CA_1 waiting itself
+--let $wait_condition = SELECT COUNT(*) = 1 FROM INFORMATION_SCHEMA.PROCESSLIST WHERE STATE = 'debug sync point: now';
+--source include/wait_condition.inc
+
 # set debug_sync="now wait_for CA_1_processing";
 connect(extra_slave,127.0.0.1,root,,test,$SLAVE_MYPORT);
 --send stop slave;