Channel Backend Test: `Verify(addr, params, state, sig)` does not verify params.

[matthiasgeihs]

Location

backend/sim/channel, backend/eth/channel

Problem

The signature verification method of the channel backend interface expects as input a channel parameters and channel state. However, our two implementations do not verify whether the given parameters are consistent with the given state. In fact, our tests were failing if an implementation is checking and throwing an error in that case. This feels counterintuitive and leads to confusion when implementing backends.

Proposal

If we provide the params via the interface, we should also verify that their are consistent with the given state. Alternatively, we may consider changing the interface definition.

[matthiasgeihs]

Addressed in #196