test: add dgram.Socket.prototype.sendto's test

Refs: https://github.com/nodejs/node/blob/09ebdf14005cc948529b3f193ad550d5dfded26a/lib/dgram.js#L234
PR-URL: https://github.com/nodejs/node/pull/10901


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>