Retesteth is a tool to run ethereum consensus tests on a client via open tcp/rpc endpoint socket. This tool take the test logic realization on itself and all a client developer has left to do is just to implement a small number of test_ rpc methods according to the instructions. (https://github.com/ethereum/retesteth/wiki/RPC-Methods)

Retesteth will run Blockchain tests via rpc and tell if your client has any issues passing the standard consensus tests from the test repo. (https://github.com/ethereum/tests/tree/develop/src)

Also if you build a different version of ethereum protocol, you could write your own tests and run it with your client using retesteth.