Two changes were made to the test:

1) Suppress warning "Refusing exit for the last slave thread."
   This warning was already suppressed, but on the wrong node.

2) The test occasionally fails because it expects that the
   number of applier threads changes immediately after
   changing the value of ```variable wsrep_slave_threads```.
   Which is not true. This patch turns snippets like this:

   ```
   SET GLOBAL wsrep_slave_threads = x;
   SELECT COUNT(*) = x FROM INFORMATION_SCHEMA.PROCESSLIST WHERE USER = 'system user';
   ```

   Into proper wait_conditions:

   ```
   SET GLOBAL wsrep_slave_threads = x;
   let $wait_condition = SELECT COUNT(*) = x FROM ...;
   --source include/wait_condition.inc
   ```