MDEV-17658 change the structure of mysql.user table

Implement User_table_json.
Fix scripts to use mysql.global_priv.
Fix tests.

mysql-test/include/mtr_check.sql

@@ -95,7 +95,7 @@ BEGIN
     mysql.time_zone_name,
     mysql.time_zone_transition,
     mysql.time_zone_transition_type,
-    mysql.user;
+    mysql.global_priv;
 
   -- verify that no plugin changed its disabled/enabled state
   SELECT * FROM INFORMATION_SCHEMA.PLUGINS;