The Test stage (aka "The Coffee Shop") is a stage that can only be accessed with an Action Replay or by installing "Crazy Mod" and is not used in the game normally.
The background of the stage has an image of Caffe Verona, Palo Alto, CA, a (now closed) coffee shop, hence its nickname. The image is one of the test images in OpenGL for testing reflections. The platforms are all gray polygons and come in different sizes. Some of these platforms are immobile, while some move vertically. The stage is very wide and stretches out at a long distance. The player can use any item, enemies, or other objects that the Debug Menu allows you to use. For some reason, the Corneria stage music will play on this level. One theory for this is related to the fact that the main tier of the stage looks similar to the Great Fox on Corneria.
The background image is actually a common test image used in 3D graphics. It can be seen in, for example, early editions of the OpenGL Programming Guide.