fix DKG test

[conradoplg]

This is a draft because it's pointing to a local change to frost-core and reddsa and won't compile for now. I'll create PR for those next.

The fix was very complicated for these reasons:

• The DKG test spawns the "main" function in a task, which requires the function to be Send. That wasn't previously required since tokio::main and tokio::test do not require functions to be Send. However, requiring Send cascades through all the program. In particular, Scalar and Element are not Send because those are traits and Send was not in their trait bounds (this will require a new PR in frost). But even fixing that, the Comms trait needs to be Send which requires a lot of the change, and the old CLI code also breaks because BufRead and Write are also not Send. This required changing all CLI code to be async which was a lot of work.
• Tokio can spawn !Send tasks with LocalSet but I couldn't get it to work. But even if it did work, anyone using frost-core with async Rust could face the same problem, so it makes sense to fix this now.