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/recorder.go

@@ -0,0 +1,54 @@
+// Package dnstest allows for easy testing of DNS client against a test server.
+package dnstest
+
+import (
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+// Recorder is a type of ResponseWriter that captures
+// the rcode code written to it and also the size of the message
+// written in the response. A rcode code does not have
+// to be written, however, in which case 0 must be assumed.
+// It is best to have the constructor initialize this type
+// with that default status code.
+type Recorder struct {
+	dns.ResponseWriter
+	Rcode int
+	Len   int
+	Msg   *dns.Msg
+	Start time.Time
+}
+
+// New makes and returns a new Recorder,
+// which captures the DNS rcode from the ResponseWriter
+// and also the length of the response message written through it.
+func NewRecorder(w dns.ResponseWriter) *Recorder {
+	return &Recorder{
+		ResponseWriter: w,
+		Rcode:          0,
+		Msg:            nil,
+		Start:          time.Now(),
+	}
+}
+
+// WriteMsg records the status code and calls the
+// underlying ResponseWriter's WriteMsg method.
+func (r *Recorder) WriteMsg(res *dns.Msg) error {
+	r.Rcode = res.Rcode
+	// We may get called multiple times (axfr for instance).
+	// Save the last message, but add the sizes.
+	r.Len += res.Len()
+	r.Msg = res
+	return r.ResponseWriter.WriteMsg(res)
+}
+
+// Write is a wrapper that records the length of the message that gets written.
+func (r *Recorder) Write(buf []byte) (int, error) {
+	n, err := r.ResponseWriter.Write(buf)
+	if err == nil {
+		r.Len += n
+	}
+	return n, err
+}