cuprate-helper: Fix RUST_BACKTRACE=1 cargo test -p cuprate-helper --lib fs::test::path_sanity_check

[An-anonymous-coder]

error: extern crate `curve25519_dalek` is unused in crate `cuprate_helper`
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  = help: remove the dependency or add `use curve25519_dalek as _;` to the crate root
  = note: requested on the command line with `-D unused-crate-dependencies`

error: extern crate `tokio` is unused in crate `cuprate_helper`
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  = help: remove the dependency or add `use tokio as _;` to the crate root

error: could not compile `cuprate-helper` (lib test) due to 2 previous errors

cargo test still fails fs::test::path_sanity_check (see #610), but this fixed something.