Extend generic channel tests

[matthiasgeihs]

Location

channel/test

Problem

There do not exist generic tests for the following interfaces: channel.Adjudicator, channel.Funder, channel.AdjudicatorSubscription. The generic backend test does not cover method DecodeAsset.

Proposal

We add generic tests for channel.Adjudicator, channel.Funder, channel.AdjudicatorSubscription, and channel.Backend.DecodeAsset.