If you put your test package in a separate folder and create a new folder next to the package called Thinstall. The test package will use this Thinstall folder as the active Sandbox location. Make sure users have modify permissions to the location.
This way there is no need for package.ini changes.
Another method is to use an environment variable, redirecting the Sandbox. But in this case I don't think it would work since it is a machine wide setting.
More details: http://pubs.vmware.com/thinapp4/help/wwhelp/wwhimpl/js/html/wwhelp.htm?href=sandbox_search_order.html