The FilterTest node is just a simple image filter example. Think of this one as a kind of "overlay." It processes any connected shaders and allows them to output to the image buffer. In other words, whatever you attach to it will be overlayed on the render (the output of Filter Test goes to the input of the Image Filter material).
Here is a sample shader flow for using the Filter Test node. If the Noise is very tiny (like 0.08), and set to Transparent, and the Opacity of the Filter Test is set low (like to 0.01), this flow can be used to add simulated film grain to the image. (Note: The Opacity is set on the Shader block.)
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