BUG#47749: rpl_slave_skip fails sporadically on PB2 (mysql-5.1-rep+2 tree).

rpl_slave_skip fails randomly on PB2. This patch fixes the failure by
setting explicit wait for SQL thread to stop, instead of the 
wait_for_slave_to_stop mysqltest command, after a start until command 
is executed.

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

@@ -27,7 +27,7 @@ connection slave;
 
 # Stop when reaching the the first table map event.
 START SLAVE UNTIL MASTER_LOG_FILE='master-bin.000001', MASTER_LOG_POS=762;
-wait_for_slave_to_stop;
+-- source include/wait_for_slave_sql_to_stop.inc
 --replace_result $MASTER_MYPORT MASTER_PORT
 --replace_column 1 # 8 # 9 # 23 # 33 # 35 # 36 #
 query_vertical SHOW SLAVE STATUS;
@@ -59,7 +59,7 @@ source include/show_binlog_events.inc;
 
 connection slave;
 START SLAVE UNTIL MASTER_LOG_FILE='master-bin.000001', MASTER_LOG_POS=106;
-wait_for_slave_to_stop;
+-- source include/wait_for_slave_sql_to_stop.inc
 SET GLOBAL SQL_SLAVE_SKIP_COUNTER=1;
 START SLAVE;
 sync_with_master;