The test used to scale the StatefulSet down to 0, wait for ListPods to
return 0 matching Pods, and then scale the StatefulSet back up.

This was prone to a race in which StatefulSet was told to scale back up
before it had observed its own deletion of the last Pod, as evidenced by
logs showing the creation of Pod ss-1 prior to the creation of the
replacement Pod ss-0.

We now wait for the controller to observe all deletions before
scaling it back up. This should fix flakes of the form:

```
Too many pods scheduled, expected 1 got 2
```