Bug#20166 mysql-test-run.pl does not test system privilege tables creation

 - Escape undercore character in mysql.db to only allow
  access test_% and not test%


scripts/mysql_system_tables.sql:
  underscore is a wildcard character so it need to be
  expaed to only match one _

scripts/mysql_system_tables.sql

@@ -8,7 +8,7 @@ CREATE TABLE IF NOT EXISTS db (   Host char(60) binary DEFAULT '' NOT NULL, Db c
 
 -- default grants for anyone to access database 'test' and 'test_%'
 INSERT INTO db VALUES ('%','test','','Y','Y','Y','Y','Y','Y','N','Y','Y','Y','Y','Y','Y','Y','Y','N','N');
-INSERT INTO db VALUES ('%','test_%','','Y','Y','Y','Y','Y','Y','N','Y','Y','Y','Y','Y','Y','Y','Y','N','N');
+INSERT INTO db VALUES ('%','test\_%','','Y','Y','Y','Y','Y','Y','N','Y','Y','Y','Y','Y','Y','Y','Y','N','N');
 
 
 CREATE TABLE IF NOT EXISTS host (  Host char(60) binary DEFAULT '' NOT NULL, Db char(64) binary DEFAULT '' NOT NULL, Select_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Insert_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Update_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Delete_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Create_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Drop_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Grant_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, References_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Index_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Alter_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Create_tmp_table_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Lock_tables_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Create_view_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Show_view_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Create_routine_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Alter_routine_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, Execute_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL, PRIMARY KEY Host (Host,Db) ) engine=MyISAM CHARACTER SET utf8 COLLATE utf8_bin comment='Host privileges;  Merged with database privileges';