Rollup merge of #89821 - crlf0710:unsafe_code_lint_test, r=Mark-Simulacrum

Add a strange test for `unsafe_code` lint.

The current behavior is a little surprising to me. I'm not sure whether people would change it, but at least let me document the current behavior with a test.

I learnt about this from the [totally-speedy-transmute](https://docs.rs/totally-speedy-transmute) crate.

cc #10599 the original implementation pr.