In my opinion that is not really a symfony related question.
Depending on your webserver you could configure an alias where which url should point.
In Apache you specify your document root, where you put your sf2 installation, and then define an alias which would be /mysymfonytest pointing to that document root.
Alternatively you could specify your document root, leave the url be and define a prefix for all routes in symfony, which you would do in your routing configuration.
I strongly recommend configuring your webserver though.
For example in apache:
<VirtualHost *:80>
DocumentRoot /var/www/mysymfonytest
Alias /mysymfonytest /var/www/mysymfonytest
<Directory /var/www/mysymfonytest/web>
AllowOverride None
Order allow,deny
Allow from all
#Allow from <SUPPORT>
RewriteEngine On
RewriteBase /mysymfonytest
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ /app.php [QSA,L]
</Directory>
</VirtualHost>
Alternatively, if you cannot add the VHost:
How to add a prefix to all my routes