fakeAsync vs await isStable

Trying to figure out which is best between:

```ts
fakeAsync(() => { 
  fixture.detectChanges();
  flush();
});
```
and
```ts
async () => {
  await fixture.whenStable();
}
```

Initial tests seem to point to the former, but the later might be cleaner as it doesn't mess with time (and allows complex test scenarios where it is necessary to manipulate time).