Consensus client tests

[nazar-pc]

We basically don't have client-side consensus tests for the whole protocol (sc-consensus-subspace), only for some of the underlying components/primitives.

The biggest challenge is the large API surface of Substrate that requires mocking of a lot of stuff. Without this infrastructure it is very hard to write tests and creating such infrastructure without running the whole node is a major undertaking.

One idea I had to address this was to introduce additional Subspace-specific abstractions that will be implemented for Substrate initially, but could be mocked much more easily for testing purposes.