Clean up th governance engine unit tests

[ValentinTrinque]

Result on code coverage from code refactoring

file	coverage before	coverage after
asset.go	0% statements	0% statements
config.go	100% statements	100% statements
engine.go	42.3% statements	79.7% statements
market.go	65.7% statements	65.7% statements
netparams.go	60.9% statements	60.9% statements
node_validation.go	5.3% statements	11.8% statements
proposal.go	22.7% statements	36.4% statements
service.go	0% statements	0% statements

Close #2011