Rollup merge of #95917 - RalfJung:thin-box-test, r=dtolnay

thin_box test: import from std, not alloc

Importing from `alloc` makes [Miri fail](https://github.com/rust-lang/miri-test-libstd/runs/5964922742?check_suite_focus=true), probably due to the hack that we used to resolve https://github.com/rust-lang/miri-test-libstd/issues/4. There might be better ways around this, but for now this is the easiest thing to do -- no other alloc integration test is importing from `alloc::`.