select user support plan v2

[TCeason]

And have a question, I try to add a test on stateless test but ther has some err: test_statements_in_legacy_suites

'root'@mysqldb 19:44:26 [(none)]> set enable_planner_v2 = 1;
Query OK, 0 rows affected (0.04 sec)
Read 0 rows, 0.00 B in 0.002 sec., 0 rows/sec., 0.00 B/sec.

'root'@mysqldb 19:44:31 [(none)]> select user();
ERROR 1105 (HY000): Code: 1005, displayText = error: 
  --> SQL:1:8
  |
1 | select user()
  | ------ ^^^^ expected `*`, `IS`, `IN`, `BETWEEN`, `+`, `-`, or 46 more ...
  | |       
  | while parsing `SELECT ...`

Originally posted by @TCeason in #5584 (comment)