pkg: add dnstest (#1098)

Add a full test server impl in this new package + tests. Move
dnsrecorder into this package as well and finish up the commented out
tests that were left in the old dnsrecorder package.

Update all callers and tests.

plugin/pkg/dnstest/server.go

@@ -0,0 +1,46 @@
+package dnstest
+
+import (
+	"net"
+
+	"github.com/miekg/dns"
+)
+
+// A Server is an DNS server listening on a system-chosen port on the local
+// loopback interface, for use in end-to-end DNS tests.
+type Server struct {
+	Addr string // Address where the server listening.
+
+	s1 *dns.Server // udp
+	s2 *dns.Server // tcp
+}
+
+// NewServer starts and returns a new Server. The caller should call Close when
+// finished, to shut it down.
+func NewServer(f dns.HandlerFunc) *Server {
+	dns.HandleFunc(".", f)
+
+	ch1 := make(chan bool)
+	ch2 := make(chan bool)
+
+	p, _ := net.ListenPacket("udp", ":0")
+	l, _ := net.Listen("tcp", p.LocalAddr().String())
+
+	s1 := &dns.Server{PacketConn: p}
+	s2 := &dns.Server{Listener: l}
+	s1.NotifyStartedFunc = func() { close(ch1) }
+	s2.NotifyStartedFunc = func() { close(ch2) }
+	go s1.ActivateAndServe()
+	go s2.ActivateAndServe()
+
+	<-ch1
+	<-ch2
+
+	return &Server{s1: s1, s2: s2, Addr: p.LocalAddr().String()}
+}
+
+// Close shuts down the server.
+func (s *Server) Close() {
+	s.s1.Shutdown()
+	s.s2.Shutdown()
+}