Improve testing around ConsensusSetSubscribe
Improve testing around ConsensusSetSubscribe

There are now tests which will complain if ConsensusSetSubscribe does
not properly remove a module from the subscribers list after a failed
subscription.

ChrisSchinnerl/Sia,MrStrong/Sia,MrStrong/Sia,mtlynch/Sia,NebulousLabs/Sia,ericflo/Sia,mnsl/Sia,mharkus/Sia,mharkus/Sia,rivine/rivine,rivine/rivine,NebulousLabs/Sia,ChrisSchinnerl/Sia,mtlynch/Sia,ericflo/Sia,mnsl/Sia