Rollup merge of #146439 - connortsui20:fix-sync-macro-attr, r=RalfJung

fix cfg for poison test macro

Fixes test regression in https://github.com/rust-lang/rust/pull/144648
Continuation of https://github.com/rust-lang/rust/pull/146433

I think this is right? Not really sure how to test this myself to be honest.

r? ```@RalfJung```

I'll also leave the improvement to the test macro for a separate PR (described [here](https://github.com/rust-lang/rust/pull/146433#issuecomment-3280210451)) since I've never done something like that before. Though since this fixes all of the tests, it might not be necessary since anyone in the future will see the `cfg()` and not `cfg_attr()`?