MDEV-33500: rpl.rpl_parallel_sbm can fail on slow machines, e.g. MSAN/Valgrind builders

In an addition to test rpl.rpl_parallel_sbm added by MDEV-32265, the
test uses sleep statements alone to test Seconds_Behind_Master with
delayed replication. On slow running machines, the test can pass the
intended MASTER_DELAY duration and Seconds_Behind_Master can become
0, when the test expects the transaction to still be actively in a
delaying state.

This can be consistently reproduced by adding a sleep statement
before the call to

--let = query_get_value(SHOW SLAVE STATUS, Seconds_Behind_Master, 1)

to sleep past the delay end point.

This patch fixes this by locking the table which the delayed
transaction targets so Second_Behind_Master cannot be updated before
the test reads it for validation.

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

@@ -27,12 +27,19 @@ connection slave;
 # delaying a transaction; then when the reciprocal START SLAVE occurs,
 # if the event is still to be delayed, SBM should resume accordingly
 include/stop_slave.inc
+# Lock t1 on slave to ensure the event can't finish (and thereby update
+# Seconds_Behind_Master) so slow running servers don't accidentally
+# catch up to the master before checking SBM.
+connection server_2;
+LOCK TABLES t1 WRITE;
 include/start_slave.inc
 connection slave;
 # Waiting for replica to resume the delay for the transaction
 # Sleeping 1s to increment SBM
 # Ensuring Seconds_Behind_Master increases after sleeping..
 # ..done
+connection server_2;
+UNLOCK TABLES;
 include/sync_with_master_gtid.inc
 #
 # Pt 2) If the worker threads have not entered an idle state, ensure