Rollup merge of #135366 - Urgau:unreach_pub-std-2, r=cuviper

Enable `unreachable_pub` lint in `test` and `proc_macro` crates

This PR enables the [`unreachable_pub`](https://doc.rust-lang.org/rustc/lints/listing/allowed-by-default.html#unreachable-pub) lint as warn in the `test` and `proc_macro` crates.

The diff was mostly generated with `./x.py fix --stage 1 library/proc_macro/ -- --broken-code`, as well as manual edits for code in macros and in tests.

Continuation of #134286

r? libs

Author: matthiaskrgr