Support cpp-ethereum's test RPC methods

[axic]

cpp-ethereum when run using --test exports a number of new RPC methods, including test_setChainParams. Full list available here.

Solidity unit tests rely on such an endpoint (together with IPC). It would be cool to use testrpc with the Solidity tests.

(Relevant ethereum/interfaces#4)

[davidmurdoch]

@axic do you think we should keep this issue around and try to renew interest in standardizing common RPC test methods? It seems even more relevant now more than ever, now that foundry, dapptools, etc are becoming popular, but it also seems like we're unlikely to get consensus on an initiative like this (seeing as you tried this in 2016 and weren't successful).