autopilot/agent_test: add TestAgentHeuristicUpdateSignal
autopilot/agent_test: add TestAgentHeuristicUpdateSignal

TestAgentHeuristicUpdateSignal tests that upon notification about a
heuristic update, the agent reconsults the heuristic.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd