There's a test suite for clients & for servers on https://indieauth.rocks/. It looks like it was built by Aaron Parecki himself. You can even see published test reports at https://indieauth.rocks/implementations.
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my own indieAuth provider! indiko.dunkirk.sh/docs
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Oh this is awesome! I found a few issue with that and now we are passing with flying colors! https://indieauth.rocks/implementations/3