`polkadot-node-core-pvf-common`: Fix test compilation error (#5310)

This crate only uses `tempfile` on linux but includes it unconditionally
in its `Cargo.toml`. It also sets `#![deny(unused_crate_dependencies)]`.
This leads to an hard error to anything that is not Linux.

This PR fixes this error. I am wondering why CI didn't catch that.
Shouldn't the test at least be compiled (but not run) on macOS?