MDEV-7251: Test failure in rpl.rpl_parallel

There was a race. The test case was expecting the slave to start processing a
particular DELETE statement, then the test would stop the slave at this
point. But there was missing something to wait until the slave would actually
reach this point; thus depending on timing it was possible that the slave
would be stopped too early, causing .result file difference.

Fixed by adding an appropriate wait to the test case.

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

@@ -983,7 +983,10 @@ SET GLOBAL slave_parallel_threads=0;
 SET GLOBAL slave_parallel_threads=10;
 INSERT INTO t2 VALUES (41);
 INSERT INTO t2 VALUES (42);
+SET @old_format= @@binlog_format;
+SET binlog_format= statement;
 DELETE FROM t2 WHERE a=40;
+SET binlog_format= @old_format;
 INSERT INTO t2 VALUES (43);
 INSERT INTO t2 VALUES (44);
 FLUSH LOGS;

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

@@ -1560,7 +1560,11 @@ SET GLOBAL slave_parallel_threads=10;
 # Setup some transaction for the slave to replicate.
 INSERT INTO t2 VALUES (41);
 INSERT INTO t2 VALUES (42);
+# Need to log the DELETE in statement format, so we can see it in processlist.
+SET @old_format= @@binlog_format;
+SET binlog_format= statement;
 DELETE FROM t2 WHERE a=40;
+SET binlog_format= @old_format;
 INSERT INTO t2 VALUES (43);
 INSERT INTO t2 VALUES (44);
 # Force the slave to switch to a new relay log file.
@@ -1581,6 +1585,11 @@ SELECT * FROM t2 WHERE a=40 FOR UPDATE;
 --connection server_2
 --source include/start_slave.inc
 
+# Wait for a worker thread to start on the DELETE that will be blocked
+# temporarily by the SELECT FOR UPDATE.
+--let $wait_condition= SELECT count(*) > 0 FROM information_schema.processlist WHERE state='updating' and info LIKE '%DELETE FROM t2 WHERE a=40%'
+--source include/wait_condition.inc
+
 # The DBUG injection set above will make the worker thread signal the following
 # debug_sync when the GTID 0-1-100 has been reached by a worker thread.
 # Thus, at this point, the SQL driver thread has reached the next