feature_fedpeg double-spend test is a bit racy

[instagibbs]

This line is triggering a "missing input" error sometimes, which means that somehow the other transaction is getting into its mempool even after being disconnected from each other a few lines before: https://github.com/ElementsProject/elements/blob/master/test/functional/feature_fedpeg.py#L629

disconnect_nodes() has a wait_until call which should ensure that they are no longer connected to each other.

[instagibbs]

#736 should solve this.