There is one part of the test case that needs to break
and re-establish the circular topology. For this the test
stops the slave threads on a couple of servers and restarts
them with START SLAVE. However, no check is done on the
status of the IO or SQL threads before proceeding with
the subsequent commands.
      
Because rpl_only_running_threads is set to 1 this can lead
to silently not syncing all slave threads as expected, 
ultimately resulting in unexpected results (and consequently
on a failing test run).
      
We fix this by replacing the START SLAVE instructions with
calls to --source include/start_slave.inc, which will wait
for the slave threads to be running (show 'Yes' in 
Slave_IO|SQL_Running fields of SHOW SLAVE STATUS) before 
proceeding. Additionally, we change rpl_sync.inc to make the
IO thread report that it is running when its running status
is any other than 'No'.