Define ChainInterface test suite template

We now have the ChainInterface type that generalizes operations performed on any chain, but we don't yet have a clear definition of how each of the methods should behave in certain circumstances.

I think it would make the most sense to write a ChainInterface "test suite template" that can function as a "working spec" for all chain implementations so that we can make their behavior as consistent and predictable as possible.

Why not just a single test suite for all implementations? Well because the initialization and send/sign/build argument types they all take differ from implementation to implementation, so some code will need to be re-written between implementations.