I feel your pain. The test suite for Handel has xml/tt output tests for its AxKit and Template Toolkit plugins. I've got oodles of template pages using the components whos output I compare to static .out files that contain the expected output. Right now, that's about 2000+ tests, with about 200 of those being the actual page output tests using all of the possible tag variations.

Everytime I write a new plugin, or a new tag in the plugin, I waste tons of time just writing the tests for them. So far, I've been good about writing the tests before I write the code, but it takes forever and I rarely get the tests right the first time.

I'm curious to see what comes out of your question. I'm in the same boat.