Rollup merge of #129123 - aDotInTheVoid:rustdoc-json-self, r=fmease

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

r? ````````@fmease````````

Author: matthiaskrgr