In both wp-config.php files, change the following defines:
define('COOKIE_DOMAIN', '.test.oursite.com');
define('COOKIEHASH', md5('test.oursite.com'));
to:
define('COOKIE_DOMAIN', '.oursite.com');
define('COOKIEHASH', md5('oursite.com'));
Go to test.oursite.com/wp-admin/ and login as an administrator. Go to Users -> Your Profile and click on Update Profile button. Now go to forums.oursite.com/wp-admin/. You should be logged in there. If synchronization plugins in mu-plugins for both sites are correct, all is done. If they are incorrect, you’ll get a message, that you don’t have privileges to access this page. In that case, you’ll have to correct synchronization plugins, according to my original answer. If you see login form, your wp-config.php files are not set correctly.