[NRY] dgram tests/improvements

the real slim shady please stand by, please stand by, please stand by
AFAICT the (undocumented!) [mis-]feature node.js has, seems to be that `new dgram.Socket('udp4')` behaves like the string version of `dgram.createSocket`. Well, okay, it doesn't seem to handle the callback either. I guess we can match this… *grumbles*