bugfix: mysql_fix_privilege_tables table_schema=database()

fix mysql_fix_privilege_tables to look for the `user` table in the
correct schema when deciding whether to convert it to `global_priv` table

make related tests a bit more verbose

Author: vuvova

mysql-test/main/system_mysql_db_fix40123.result

@@ -760,7 +760,7 @@ ALTER TABLE column_stats ENGINE=Aria transactional=0;
 ALTER TABLE index_stats ENGINE=Aria transactional=0;
 
 DELIMITER //
-IF 'BASE TABLE' = (select table_type from information_schema.tables where table_name='user') THEN
+IF 'BASE TABLE' = (select table_type from information_schema.tables where table_schema=database() and table_name='user') THEN
   CREATE TABLE IF NOT EXISTS global_priv (Host char(60) binary DEFAULT '', User char(80) binary DEFAULT '', Priv JSON NOT NULL DEFAULT '{}' CHECK(JSON_VALID(Priv)), PRIMARY KEY Host (Host,User)) engine=Aria transactional=1 CHARACTER SET utf8 COLLATE utf8_bin comment='Users and global privileges'
   SELECT Host, User, JSON_COMPACT(JSON_OBJECT('access',
                              1*('Y'=Select_priv)+