rustdoc-json: Add test for `Self` type
Inspired by #128471, the rustdoc-json suite had no tests in place for the `Self` type. This PR adds one.
I've also manually checked locally that this test passes on 29e924841f06bb181d87494eba2783761bc1ddec, confirming that adding `clean::Type::SelfTy` didn't change the JSON output. (potentially adding a self type to json (insead of (ab)using generic) is tracked in #128522)
Updates #81359
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