Optionalize [channel] GenericBackendTest

[ggwpez]

Current situation: GenericBackendTest assumes that the signature of a state depends on all fields.
The test now fails as we create new backends where the adjudicator does not care about all fields.

Possible solution: Add some options like NoAppData, NoLocked… and pass them to GenericBackendTest.
The test would then treat those as Don’t-Care.