I just realized that there is no good place for adding
exposed function or method tests because server, unicast
and multicast_test.go do test complicated multiple test
objects, platform behaviros, protocol behaviors and API,
at the same time. Perhaps splitting them into per test
object might be better, so this CL provides tests focused
on API.

R=rsc
CC=gobot, golang-dev
https://golang.org/cl/6501057