test: Move testing couriers to testlib

The testing couriers are currently only used in the queue tests, but we
also want to use them in smtpsrv tests so we can make them more robusts
by checking the emails got delivered.

This patch moves the testing couriers to testlib, and makes both queue
and smtpsrv use them.

internal/testlib/testlib.go

@@ -6,7 +6,9 @@ import (
 	"net"
 	"os"
 	"strings"
+	"sync"
 	"testing"
+	"time"
 )
 
 // MustTempDir creates a temporary directory, or dies trying.
@@ -64,3 +66,62 @@ func GetFreePort() string {
 	defer l.Close()
 	return l.Addr().String()
 }
+
+func WaitFor(f func() bool, d time.Duration) bool {
+	start := time.Now()
+	for time.Since(start) < d {
+		if f() {
+			return true
+		}
+		time.Sleep(20 * time.Millisecond)
+	}
+	return false
+}
+
+type DeliverRequest struct {
+	From string
+	To   string
+	Data []byte
+}
+
+// Courier for test purposes. Never fails, and always remembers everything.
+type TestCourier struct {
+	wg       sync.WaitGroup
+	Requests []*DeliverRequest
+	ReqFor   map[string]*DeliverRequest
+	sync.Mutex
+}
+
+func (tc *TestCourier) Deliver(from string, to string, data []byte) (error, bool) {
+	defer tc.wg.Done()
+	dr := &DeliverRequest{from, to, data}
+	tc.Lock()
+	tc.Requests = append(tc.Requests, dr)
+	tc.ReqFor[to] = dr
+	tc.Unlock()
+	return nil, false
+}
+
+func (tc *TestCourier) Expect(i int) {
+	tc.wg.Add(i)
+}
+
+func (tc *TestCourier) Wait() {
+	tc.wg.Wait()
+}
+
+// NewTestCourier returns a new, empty TestCourier instance.
+func NewTestCourier() *TestCourier {
+	return &TestCourier{
+		ReqFor: map[string]*DeliverRequest{},
+	}
+}
+
+type dumbCourier struct{}
+
+func (c dumbCourier) Deliver(from string, to string, data []byte) (error, bool) {
+	return nil, false
+}
+
+// Dumb courier, for when we just don't care about the result.
+var DumbCourier = dumbCourier{}