Fix galera_log_output_csv.test

Truncate the mysql.slow_log table after test is complete.

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

@@ -16,3 +16,5 @@ SELECT COUNT(*) = 1 FROM mysql.slow_log WHERE sql_text = 'SELECT 2 = 2 FROM t1';
 COUNT(*) = 1
 1
 DROP TABLE t1;
+truncate table mysql.slow_log;
+truncate table mysql.general_log;

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

@@ -22,3 +22,5 @@ SELECT COUNT(*) = 1 FROM mysql.slow_log WHERE sql_text = 'SELECT 2 = 2 FROM t1';
 
 --connection node_1
 DROP TABLE t1;
+truncate table mysql.slow_log;
+truncate table mysql.general_log;