SQL: derived SYSTEM_TIME clash detection [closes #371]

mysql-test/suite/versioning/t/cte.test

@@ -49,7 +49,23 @@ as
        ancestors as a
   where e.mgr = a.emp_id
 )
-select * from ancestors for system_time as of current_timestamp;
+select * from ancestors;
+
+with recursive
+ancestors
+as
+(
+  select e.emp_id, e.name, e.mgr, e.salary
+  from emp as e
+  where name = 'bill'
+  union
+  select e.emp_id, e.name, e.mgr, e.salary
+  from emp as e,
+       ancestors as a
+  where e.mgr = a.emp_id
+)
+select * from ancestors
+for system_time as of timestamp @ts_1;
 
 /* Expected 3 rows */
 with recursive