Question: adding a test, how-to?

I think I found a bug in Aeson. I want to contribute a fix. Before the fix, I want to prove that the bug exists with a failing unit test. How should do it? Should that regression test be a golden test? Some other test? Which modules I should look at first. Please advice :) I will be also happy to contribute a "How to contribute" guide for future generations to come :)

The issue names no files, tests, or modules to start from. Review Aeson's existing testing conventions to determine when a regression should use a golden test versus another unit-test style, and identify the relevant modules. Done means providing clear contribution and regression-testing guidance, possibly as a future How to Contribute guide.

Written by the indexing model from the issue text.