Fix termination bug

[smklein]

Fix a return not returning enough.

TL:DR: The following code loops forever, because return returns from the async block, not from foobar.

async fn foobar() {
  loop {
     async {
        return;
     }.await;
  }
}

This issue impacted slot teardown, and could result in re-using a oneshot::Receiver.

This PR fixes that issue, and adds additional tests for termination (which fail without the patch).

[smklein]

In short: The bug is here.

[smklein]

Gonna merge this aggressively to see if it helps oxidecomputer/omicron#6881 , but I wanted to give you a heads-up -- this feels like a very silly mistake I made