[dns] tests take a long time to run

[rade]

$ go test -tags netgo github.com/weaveworks/weave/nameserver
ok      github.com/weaveworks/weave/nameserver  9.072s

Adding some debug logging and running the tests with -v reveals that most of that time is spent in shutting down dns servers. That turns out due to the way Shutdown is implemented in the DNS library we are using. The server sits in a loop that reads with a timeout and checks whether it should shut down every time the read returns. The default timeout is 2 seconds. This can be changed by setting Server.ReadTimeout.