[minor] replace libtest/lib.rs:FnBox by std::boxed::FnBox

[Yamakaky]

https://github.com/rust-lang/rust/blob/master/src/libtest/lib.rs#L134-L142

https://doc.rust-lang.org/std/boxed/trait.FnBox.html

I can do it if wanted

[GuillaumeGomez]

Then go ahead. :)

[Yamakaky]

Waiting for #41815 to be merged