chore(dependencies): enable futures-util/alloc for tests

[seanmonstar]

Closes #3599

[seanmonstar]

Hm, so the syntax to enable this on an optional feature works in newest Cargo, but the Cargo from 1.63 only understands half of it:

does not have feature futures-util. It has an optional dependency with that name, but that dependency uses the "dep:" syntax in the features table, so it does not have an implicit feature with that name.

Using ffi = ["dep:futures-util/alloc"] causes an error in all Cargo versions. How did one do this before it was "fixed"? Is Rust 1.63 just left out in the rain?

[seanmonstar]

Following advice in this issue: rust-lang/cargo#10788

Changed to be futures-util?/alloc. 🤷