Allow `waitUntil` to accept a callback that returns a promise

Right now, `waitUntil` expects that the callback returns a truthy/falsy value. It would be nice to allow the callback to return a promise which resolves to a truthy/falsy value. So something like to work (and return after 1 second not immediately like now because the returned promise is a truthy value):

```typescript
await waitUntil(() => new Promise((resolve) => setTimeout(() => resolve(true), 1000)));
```