Testing async/await middleware?
We're looking to migrate over to Koa 2. So far i've converted from generators to async await but I'm having a hard time wrapping my head around how to update my unit tests to test context bubbling up. I've scoured your code and other repo's seeing if anyone has good middleware examples but come up short. 

I used to test my middleware as such:
```js
const gen = someMiddleware.call(koaContext);
// Here I force bubble up behavior allowing my middleware to add 
// or manipulate data on the context.
gen.next().next();

koaContext.state.someValue.should.equal(...)
```

How do I accomplish the same bubble up behavior using `async await`?