Add assets for api docs to test registry

[IvoneDjaja]

Fixes flutter/flutter#112116

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[Piinks]

Hey @IvoneDjaja thanks so much for adding this and for flutter/assets-for-api-docs#264 so that it runs silently as needed here. I am going to recommend we don't land these changes though. The original issue is a few years old now, and since then, we have not been using or maintaining the assets-for-api-docs repo as much as we used to. Given its status, I would not want it to block changes in flutter/flutter, given we don't have anyone actively working on it right now. I have closed the issue to reflect this now, but thank you very much for contributing!

[IvoneDjaja]

Alright @Piinks , I will close the PR then