test: Fix flaky courier test

In the upcoming Go release, logging from a finished testing.T triggers a
panic.  In the courier tests, this is possible because we don't wait for
completion of fakeServer before ending the test.

This patch makes the tests wait for fakeServer to finish before exiting,
removing the race.

internal/courier/smtp_test.go

@@ -6,6 +6,7 @@ import (
 	"net"
 	"net/textproto"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -39,13 +40,17 @@ func newSMTP(t *testing.T) (*SMTP, string) {
 }
 
 // Fake server, to test SMTP out.
-func fakeServer(t *testing.T, responses map[string]string) string {
+func fakeServer(t *testing.T, responses map[string]string) (string, *sync.WaitGroup) {
 	l, err := net.Listen("tcp", "localhost:0")
 	if err != nil {
 		t.Fatalf("fake server listen: %v", err)
 	}
 
+	wg := &sync.WaitGroup{}
+	wg.Add(1)
+
 	go func() {
+		defer wg.Done()
 		defer l.Close()
 
 		c, err := l.Accept()
@@ -79,7 +84,7 @@ func fakeServer(t *testing.T, responses map[string]string) string {
 		}
 	}()
 
-	return l.Addr().String()
+	return l.Addr().String(), wg
 }
 
 func TestSMTP(t *testing.T) {
@@ -96,7 +101,7 @@ func TestSMTP(t *testing.T) {
 		"_DATA":             "250 data ok\n",
 		"QUIT":              "250 quit ok\n",
 	}
-	addr := fakeServer(t, responses)
+	addr, wg := fakeServer(t, responses)
 	host, port, _ := net.SplitHostPort(addr)
 
 	// Put a non-existing host first, so we check that if the first host
@@ -114,6 +119,8 @@ func TestSMTP(t *testing.T) {
 	if err != nil {
 		t.Errorf("deliver failed: %v", err)
 	}
+
+	wg.Wait()
 }
 
 func TestSMTPErrors(t *testing.T) {
@@ -163,7 +170,7 @@ func TestSMTPErrors(t *testing.T) {
 	}
 
 	for _, rs := range responses {
-		addr := fakeServer(t, rs)
+		addr, wg := fakeServer(t, rs)
 		host, port, _ := net.SplitHostPort(addr)
 
 		testMX["to"] = []*net.MX{{Host: host, Pref: 10}}
@@ -176,6 +183,8 @@ func TestSMTPErrors(t *testing.T) {
 			t.Errorf("deliver not failed in case %q: %v", rs["_welcome"], err)
 		}
 		t.Logf("failed as expected: %v", err)
+
+		wg.Wait()
 	}
 }