Adds an integrated routing test of probability extrapolation for untried
channels. The larger part of this commit is mock code to simulate the
Lightning Network.
The difference between this test and the existing pathfinding tests, is that
this test focuses on the feedback loop from result interpretation via
mission control updates and probability estimation back to pathfinding.
Improvements like probability extrapolation were previously only
validated by reasoning, while this setup makes it possible to assert the
improvement in a test and guard it for the future.
