Is disabling test_form in wp_handle_upload a security concern?

I wouldn’t say security concern – since you’re using this plugin within the admin (I presume after the user has been authenticated i.e. after admin_init), then already you’ve got protection against any Jon Doe posting a form to your script.

What it does provide, is an additional layer of authentication. All it really boils down to is sending a ‘secret’ along with the form, and then checking for it’s existence (and that it matches) before continuing.

For an attacker, they’d need to know this secret in order to breach, for example, using CSRF or XSS.

This is the very nature of how WordPress nonces work. In fact you’d be a lot better off using these instead of test_form. They go one step better in that they’re secrets that expire, so the window for an attacker is made even smaller.

Check out Jaquith’s article on Nonces.