Figure out long-term plan for testing Client

[matklad]

Today, our testing story for the client is inconsistent.

On the one hand, client is isolated from the runtime via RuntimeAdapter trait and has a simple KeyValue runtime specifically for tests.

On the other hand, due to https://github.com/near/nearcore/issues/6910, many tests can't be run with KeyValue runtime, and require full runtime.

We should move to either:

• removing KeyValue runtime and just running all tests with real runtime
• moving epoch manager from runtime to chain and running all tests with KeyValue runtime

[matklad]

Discussed this a bit, current thinking:

• wait until we move epoch manager to chain, than revisit
• @matklad's prior is that the eventual soln is going to be "use real runtime"