MDEV-22040 versioning.truncate_privilege : GRANT fails with "Access denied", for root user

main.mysql_upgrade_noengine did not do "FLUSH PRIVILEGES" after restoring
the original backed global_priv table. So following tests might fail on
lack of some privileges.

Adding the FLUSH PRIVILEGES statement.

mysql-test/main/mysql_upgrade_noengine.result

@@ -315,3 +315,7 @@ uninstall plugin blackhole;
 uninstall plugin archive;
 drop table mysql.global_priv;
 rename table mysql.global_priv_bak to mysql.global_priv;
+FLUSH PRIVILEGES;
+SHOW GRANTS FOR root@localhost;
+Grants for root@localhost	GRANT ALL PRIVILEGES ON *.* TO `root`@`localhost` WITH GRANT OPTION
+Grants for root@localhost	GRANT PROXY ON ''@'%' TO 'root'@'localhost' WITH GRANT OPTION

mysql-test/main/mysql_upgrade_noengine.test

@@ -76,3 +76,6 @@ uninstall plugin archive;
 
 drop table mysql.global_priv;
 rename table mysql.global_priv_bak to mysql.global_priv;
+
+FLUSH PRIVILEGES;
+SHOW GRANTS FOR root@localhost;