This test also introduces additional round-trip tests for small files

Without this change you need to do:

```
User user;
final SecurityContext oldContext = ACL.impersonate(a);
try {
    user = User.current();
} finally {
    SecurityContextHolder.setContext(oldContext);
}
```

Which seems incredibly wasteful.

Or else you need to replicate the logic in User.current() which makes that code hard to change.

This reverts commit fe1a1487.
It's been about a month, so I think it has served its purpose.

Allowing some methods to be overridden, with the caveat that the overrides will not be called from CoreWrapperStep.

[FIXED JENKINS-20989] PeepholePermalink RunListenerImpl oncompleted should be triggered before downstream builds are triggered

 - queueLengthSnapshot never used.