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/dkim/sign_test.go

@@ -1,7 +1,6 @@
 package dkim
 
 import (
-	"context"
 	"crypto/ecdsa"
 	"crypto/ed25519"
 	"crypto/elliptic"
@@ -33,7 +32,7 @@ Joe.
 `)
 
 func TestSignRSA(t *testing.T) {
-	ctx := context.Background()
+	ctx := t.Context()
 	ctx = WithTraceFunc(ctx, t.Logf)
 
 	// Generate a new key pair.
@@ -84,7 +83,7 @@ func TestSignRSA(t *testing.T) {
 }
 
 func TestSignEd25519(t *testing.T) {
-	ctx := context.Background()
+	ctx := t.Context()
 	ctx = WithTraceFunc(ctx, t.Logf)
 
 	// Generate a new key pair.
@@ -141,7 +140,7 @@ func TestSignBadMessage(t *testing.T) {
 		Domain:   "example.com",
 		Selector: "test",
 	}
-	_, err := s.Sign(context.Background(), "Bad message")
+	_, err := s.Sign(t.Context(), "Bad message")
 	if err == nil {
 		t.Errorf("Sign: wanted error; got nil")
 	}
@@ -159,7 +158,7 @@ func TestSignBadAlgorithm(t *testing.T) {
 	}
 	s.Signer = priv
 
-	_, err = s.Sign(context.Background(), basicMessage)
+	_, err = s.Sign(t.Context(), basicMessage)
 	if !errors.Is(err, errUnsupportedKeyType) {
 		t.Errorf("Sign: wanted unsupported key type; got %v", err)
 	}