Added flush tables to acl_load_mutex-5170.test

This is needed as otherwise a lot of MyISAM tables may be marked
as crashed, depending on in which order tests are run

mysql-test/suite/roles/acl_load_mutex-5170.result

 create user user1@localhost;
 create role r1 with admin user1@localhost;
 grant all on test.* to r1;
+flush tables;
 select 1;
 1
 1

mysql-test/suite/roles/acl_load_mutex-5170.test

@@ -5,6 +5,7 @@
 create user user1@localhost;
 create role r1 with admin user1@localhost;
 grant all on test.* to r1;
+flush tables;
 
 --append_file $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
 wait