Dynamically set debug flag for session only

- avoid restart
- keep all in one file
- fix --check-testcase

mysql-test/r/error_simulation.result

@@ -13,6 +13,7 @@ INSERT INTO t1 VALUES
 ('AAAAAAAAAH','AAAAAAAAAH'), ('AAAAAAAAAI','AAAAAAAAAI'),
 ('AAAAAAAAAJ','AAAAAAAAAJ'), ('AAAAAAAAAK','AAAAAAAAAK');
 set tmp_table_size=1024;
+set session debug="d,raise_error";
 SELECT MAX(a) FROM t1 GROUP BY a,b;
 ERROR 23000: Can't write; duplicate key in table 'tmp_table'
 set tmp_table_size=default;