What app tests all of GTK's UI elements?

For GTK+3 you have the official gtk3-widget-factory in package gtk-3-examples.

"The widget factory" doesn't support Gtk3. For that you can use "A widget factory":

https://github.com/valr/awf

You can install it from the developer's PPA by typing in a terminal:

sudo add-apt-repository ppa:valere-monseur/version

sudo apt-get update && apt-get install awf

Depending on your platform, replace version with either oneiric, precise or quantal.