dnstest: add multirecorder (#1326)

* dnstest: add multirecorder

This adds a new recorder that captures all messages written to it. This
can be useful when, for instance, testing AXFR which can write muliple
messages back to the client.

* docs

plugin/pkg/dnstest/multirecorder_test.go

@@ -0,0 +1,39 @@
+package dnstest
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestMultiWriteMsg(t *testing.T) {
+	w := &responseWriter{}
+	record := NewMultiRecorder(w)
+
+	responseTestName := "testmsg.example.org."
+	responseTestMsg := new(dns.Msg)
+	responseTestMsg.SetQuestion(responseTestName, dns.TypeA)
+
+	record.WriteMsg(responseTestMsg)
+	record.WriteMsg(responseTestMsg)
+
+	if len(record.Msgs) != 2 {
+		t.Fatalf("Expected 2 messages to be written, but instead found %d\n", len(record.Msgs))
+
+	}
+	if record.Len != responseTestMsg.Len()*2 {
+		t.Fatalf("Expected the bytes written counter to be %d, but instead found %d\n", responseTestMsg.Len()*2, record.Len)
+	}
+}
+
+func TestMultiWrite(t *testing.T) {
+	w := &responseWriter{}
+	record := NewRecorder(w)
+	responseTest := []byte("testmsg.example.org.")
+
+	record.Write(responseTest)
+	record.Write(responseTest)
+	if record.Len != len(responseTest)*2 {
+		t.Fatalf("Expected the bytes written counter to be %d, but instead found %d\n", len(responseTest)*2, record.Len)
+	}
+}