Introduce traits to make test utils generic across the `CM` Holder

Introduce traits to make test utils generic across the `CM` Holder

In our test utilities, we generally refer to a `Node` struct which
holds a `ChannelManager` and a number of other structs. However, we
use the same utilities in benchmarking, where we have a different
`Node`-like struct. This made moving from macros to functions
entirely impossible, as we end up needing multiple types in a given
context.

Thus, here, we take the pain and introduce some wrapper traits
which encapsulte what we need from `Node`, swapping some of our
macros to functions.