MDEV-7241: rpl.rpl_parallel2 fails sporadically in buildbot

There was a race, a small window between updating slave position and updating
Seconds_Behind_Master, during which the test case could see the wrong value.

Fix by waiting for the expected status to appear.

mysql-test/suite/rpl/r/rpl_parallel2.result

@@ -9,6 +9,7 @@ CALL mtr.add_suppression("Unsafe statement written to the binary log using state
 INSERT INTO t1 VALUES (1,sleep(2));
 Warnings:
 Note	1592	Unsafe statement written to the binary log using statement format since BINLOG_FORMAT = STATEMENT. Statement is unsafe because it uses a system function that may return a different value on the slave.
+include/wait_for_slave_param.inc [Seconds_Behind_Master]
 Seconds_Behind_Master should be zero here because the slave is fully caught up and idle.
 Seconds_Behind_Master = '0'
 include/stop_slave.inc

mysql-test/suite/rpl/t/rpl_parallel2.test

@@ -25,6 +25,13 @@ INSERT INTO t1 VALUES (1,sleep(2));
 --connection server_2
 --sync_with_master
 
+# The slave position (which --sync_with_master waits for) is updated just
+# before the Seconds_Behind_Master. So we have to wait for the zero status
+# to appear, otherwise there is a small window between --sync_with_master
+# and SHOW SLAVE STATUS where we can see a non-zero value.
+--let $slave_param= Seconds_Behind_Master
+--let $slave_param_value= 0
+--source include/wait_for_slave_param.inc
 --echo Seconds_Behind_Master should be zero here because the slave is fully caught up and idle.
 --let $status_items= Seconds_Behind_Master
 --source include/show_slave_status.inc