Add asserts for `UnwindSafe` and `RefUnwindSafe` in `async_send_sync.rs`

[Darksonn]

To avoid unintentionally changing auto traits, we have a test called tokio/tests/async_send_sync.rs that lists the auto traits for every public type. However, this test does not mention the two unwind safety auto traits, so they should be added.

[SimonVervisch]

As far as I see, the auto traits checked at the moment are:

• unpin
• Send
• Sync

Is that correct?

[Darksonn]

Yes.