Updated benchmark and added tests

[MarkReedZ]

I moved jsonrpc_client.go to test/bench.go and added support for multiple connections via -c.

I've also added test/test.go to do a few simple checks

On ubuntu 24.04 with posix and uring I see:

$ go run test/test.go
  Test rpc ... successful
  Test html ... unexpected reply
    exp:  {"jsonrpc":"2.0","id":0,"result":true}
    act:  {"jsonrpc":"2.0","id":0,"error":{"code":-32700,"message":"Invalid JSON was received by the server."}}
  Test 4097 byte json ... successful
  Test partial ... successful