Updated privileges management

In MySQL 5.7.9 there is a new user defined by default (mysql.sys) which
is needed for the sys schema. The privilege tables are no longer cleaned
by sandbox management: this is no longer necessary, since there are no
anonymous accounts.
The test has been updated to reflect the changes

lib/MySQL/Sandbox/Scripts.pm

@@ -1509,9 +1509,9 @@ GRANTS_MYSQL
 
 use mysql;
 set password='_DBPASSWORD_';
-delete from tables_priv;
-delete from columns_priv;
-delete from db;
+-- delete from tables_priv;
+-- delete from columns_priv;
+-- delete from db;
 delete from user where user not in ('root', 'mysql.sys');
 
 flush privileges;

t/user_privileges.sb.pl

@@ -21,10 +21,9 @@
 
 if ( (($major ==5) &&  ($minor > 7) )
         or 
-    ( ($major ==5) && ($minor == 7) && ($rev > 7) ) 
+    ( ($major ==5) && ($minor == 7) && ($rev > 8) ) 
    )
-# Starting with MySQL 5.7.8 we have the sys schema included with MySQL.
-# This requires an extra user
+# Starting with MySQL 5.7.9 we have the mysql.sys user created by default.
 {
    $number_of_users = 9;
 }