Fix galera_kill_applier test case.

mysql-test/suite/galera/r/galera_kill_applier.result

 connection node_1;
+SET GLOBAL wsrep_slave_threads=2;
 Got one of the listed errors
 Got one of the listed errors
 Got one of the listed errors

mysql-test/suite/galera/t/galera_kill_applier.test

@@ -7,8 +7,10 @@
 --source include/force_restart.inc
 
 --connection node_1
---let $wait_condition = SELECT COUNT(*) = 1 FROM INFORMATION_SCHEMA.PROCESSLIST WHERE USER = 'system user' AND STATE = 'wsrep applier idle';
---let $wait_condition_on_error_output = SELECT COUNT(*), 1 as EXPECTED_VALUE FROM INFORMATION_SCHEMA.PROCESSLIST WHERE USER = 'system user' AND STATE = 'wsrep applier idle'; show processlist
+SET GLOBAL wsrep_slave_threads=2;
+
+--let $wait_condition = SELECT COUNT(*) = 2 FROM INFORMATION_SCHEMA.PROCESSLIST WHERE USER = 'system user' AND STATE = 'wsrep applier idle';
+--let $wait_condition_on_error_output = SELECT COUNT(*), 2 as EXPECTED_VALUE FROM INFORMATION_SCHEMA.PROCESSLIST WHERE USER = 'system user' AND STATE = 'wsrep applier idle'; show processlist
 --source include/wait_condition_with_debug.inc
 
 --let $applier_thread = `SELECT ID FROM INFORMATION_SCHEMA.PROCESSLIST WHERE USER = 'system user' AND STATE = 'wsrep applier idle' LIMIT 1`