test: Use modern testing features instead of custom helpers

Many of our tests rely on helpers that predate features which are now
available in the standard library.

This patch replaces them with the standard equivalents:

- MustTempDir/RemoveIfOk -> t.ArtifactDir() or t.TempDir() (depending on
  whether it is useful to keep the data on request).
- GetFreePort + waitForServer -> opening the listeners before starting
  the servers. To support this, localrpc.Server gains a Serve method. We
  may re-introduce these in the future if more use cases appear.
- WaitFor and real-time timing checks -> testing/synctest.
- context.Background() -> t.Context().
- Benchmark b.N loops -> b.Loop().
- reflect.DeepEqual -> cmp.Diff, and proto.Equal for protobufs.

internal/smtpsrv/server_test.go

@@ -12,7 +12,6 @@ import (
 	"os"
 	"strings"
 	"testing"
-	"time"
 
 	"blitiri.com.ar/go/chasquid/internal/aliases"
 	"blitiri.com.ar/go/chasquid/internal/auth"
@@ -491,8 +490,7 @@ func TestAddDKIMSigner(t *testing.T) {
 		t.Errorf("AddDKIMSigner: expected not exist, got %v", err)
 	}
 
-	tmpDir := testlib.MustTempDir(t)
-	defer testlib.RemoveIfOk(t, tmpDir)
+	tmpDir := t.ArtifactDir()
 
 	// Invalid PEM file.
 	kf1 := tmpDir + "/key1-bad_pem.pem"
@@ -555,8 +553,7 @@ func BenchmarkManyEmails(b *testing.B) {
 	c := mustDial(b, ModeSMTP, false)
 	defer c.Close()
 
-	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		sendEmail(b, c)
 	}
 }
@@ -576,25 +573,6 @@ func BenchmarkManyEmailsParallel(b *testing.B) {
 // === Test environment ===
 //
 
-// waitForServer waits 5 seconds for the server to start, and returns an error
-// if it fails to do so.
-// It does this by repeatedly connecting to the address until it either
-// replies or times out. Note we do not do any validation of the reply.
-func waitForServer(addr string) error {
-	start := time.Now()
-	for time.Since(start) < 10*time.Second {
-		conn, err := net.Dial("tcp", addr)
-		if err == nil {
-			conn.Close()
-			return nil
-		}
-
-		time.Sleep(100 * time.Millisecond)
-	}
-
-	return fmt.Errorf("not reachable")
-}
-
 type brokenAuthBE struct{}
 
 func (b brokenAuthBE) Authenticate(user, password string) (bool, error) {
@@ -615,11 +593,7 @@ func realMain(m *testing.M) int {
 	flag.Parse()
 
 	// Create a 1MiB string, which the large message tests use.
-	buf := make([]byte, 1024*1024)
-	for i := range buf {
-		buf[i] = 'a'
-	}
-	str1MiB = string(buf)
+	str1MiB = strings.Repeat("a", 1024*1024)
 
 	// Set up the mail log to stdout, which is captured by the test runner,
 	// so we have better debugging information on failures.
@@ -647,17 +621,29 @@ func realMain(m *testing.M) int {
 			return 1
 		}
 
-		smtpAddr = testlib.GetFreePort()
-		submissionAddr = testlib.GetFreePort()
-		submissionTLSAddr = testlib.GetFreePort()
-
 		s := NewServer()
 		s.Hostname = "localhost"
 		s.MaxDataSize = int64(maxDataSizeMiB) * 1024 * 1024
 		s.AddCerts(tmpDir+"/cert.pem", tmpDir+"/key.pem")
-		s.AddAddr(smtpAddr, ModeSMTP)
-		s.AddAddr(submissionAddr, ModeSubmission)
-		s.AddAddr(submissionTLSAddr, ModeSubmissionTLS)
+
+		// Open the listeners ourselves, so the ports are already bound
+		// (and we know their addresses) by the time the tests run.
+		for _, l := range []struct {
+			addr *string
+			mode SocketMode
+		}{
+			{&smtpAddr, ModeSMTP},
+			{&submissionAddr, ModeSubmission},
+			{&submissionTLSAddr, ModeSubmissionTLS},
+		} {
+			ln, err := net.Listen("tcp", "localhost:0")
+			if err != nil {
+				fmt.Printf("Failed to listen: %v\n", err)
+				return 1
+			}
+			*l.addr = ln.Addr().String()
+			s.AddListeners([]net.Listener{ln}, l.mode)
+		}
 
 		s.InitQueue(tmpDir+"/queue", localC, remoteC)
 
@@ -687,9 +673,6 @@ func realMain(m *testing.M) int {
 		go s.ListenAndServe()
 	}
 
-	waitForServer(smtpAddr)
-	waitForServer(submissionAddr)
-	waitForServer(submissionTLSAddr)
 	return m.Run()
 }