Add capabilities to generate and run test fixtures using JSON-RPC

[wtf]

We should be able to generate and execute client tests using clients other than aleth (current implementation can be found at https://github.com/ethereum/testeth). Adding json-rpc endpoints to clients in order to implement a standardized way of doing these seems like the way to go.

The goal here is to add these capabilities to Trinity, so it can serve as a starting point (and eventually, maybe a reference implementation) for all clients to implement them.

A high-level to-do list might be:

• Add functionality to execute test artefacts
• Add functionality to generate test artefacts from fillers
• Expose testing functionality using json-rpc
• Ensure all functions needed for testing are exposed through json-rpc by clients
• Draft a spec for genesis blocks (Common genesis.json format scheme across all client implementations EIPs#1085)
• Draft a spec for json-rpc endpoints used for testing

Related work so far:

• https://github.com/ethereum/retesteth
• Implement all JSON-RPC endpoints #26

Related issues:

• Implement testing JSON RPC APIs for retesteth #35

Useful resources:

• Common genesis.json format scheme across all client implementations EIPs#1085
• Document JSON-RPC interface in an EIP EIPs#1442