Shelley tests must not use FakeVRF in the leadership check

I'm labeling this as `testing` for now. However, this is urgent because we're not yet sure if it's localized to the tests. I'll update this description once we know more. (Edit: I believe it is localized to the tests.)

When the OBFT overlay is in control, active slots have one leader. But when Praos determines leaders, every active slot is lead by all nodes. Specifically, the argument to `checkLeaderValue` called from `checkIsLeader` is the same for all nodes (as observed via `Debug.Trace`).

cc: @edsko @mrBliss @nc6