Test galera.MW-366 is not deterministic and depends on timing assumptions.
The test occasionally fails after checking the number of 'system user'
processes in processlist after changing the value of variable global
wsrep_slave_threads, like this:
```
SET GLOBAL wsrep_slave_threads = x;
--sleep 0.5
SELECT COUNT(*) = x FROM INFORMATION_SCHEMA.PROCESSLIST WHERE USER = 'system user';
```

The problem is that the number of slave threads is internally adjusted
'asynchronously', and it may take some time to spawn/kill new threads,
especially in a heavily loaded system.

This patch removes the '--sleep 0.5' statements from the test and replaces
those with appropriate wait conditions, like this:
```
SET GLOBAL wsrep_slave_threads = x;
let $wait_condition = SLECT COUNT(*) = x FROM ...;
--source include/wait_condition.inc
```