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/userdb/userdb_test.go

@@ -1,34 +1,21 @@
 package userdb
 
 import (
-	"fmt"
 	"os"
-	"reflect"
-	"strings"
 	"testing"
+
+	"google.golang.org/protobuf/proto"
 )
 
-// Remove the file if the test was successful. Used in defer statements, to
-// leave files around for inspection when the tests failed.
-func removeIfSuccessful(t *testing.T, fname string) {
-	// Safeguard, to make sure we only remove test files.
-	// This should help prevent accidental deletions.
-	if !strings.Contains(fname, "userdb_test") {
-		panic("invalid/dangerous directory")
-	}
-
-	if !t.Failed() {
-		os.Remove(fname)
-	}
-}
-
 // Create a database with the given content on a temporary filename. Return
 // the filename, or an error if there were errors creating it.
 func mustCreateDB(t *testing.T, content string) string {
-	f, err := os.CreateTemp("", "userdb_test")
+	t.Helper()
+	f, err := os.CreateTemp(t.ArtifactDir(), "userdb_test")
 	if err != nil {
 		t.Fatal(err)
 	}
+	defer f.Close()
 
 	if _, err := f.WriteString(content); err != nil {
 		t.Fatal(err)
@@ -39,22 +26,7 @@ func mustCreateDB(t *testing.T, content string) string {
 }
 
 func dbEquals(a, b *DB) bool {
-	if a.db == nil || b.db == nil {
-		return a.db == nil && b.db == nil
-	}
-
-	if len(a.db.Users) != len(b.db.Users) {
-		return false
-	}
-
-	for k, av := range a.db.Users {
-		bv, ok := b.db.Users[k]
-		if !ok || !reflect.DeepEqual(av, bv) {
-			return false
-		}
-	}
-
-	return true
+	return proto.Equal(a.db, b.db)
 }
 
 var emptyDB = &DB{
@@ -80,7 +52,6 @@ func TestEmptyLoad(t *testing.T) {
 
 func testOneLoad(t *testing.T, desc, content string, fatal bool, fatalErr error) {
 	fname := mustCreateDB(t, content)
-	defer removeIfSuccessful(t, fname)
 	db, err := Load(fname)
 	if fatal {
 		if err == nil {
@@ -109,7 +80,6 @@ func mustLoad(t *testing.T, fname string) *DB {
 
 func TestWrite(t *testing.T) {
 	fname := mustCreateDB(t, "")
-	defer removeIfSuccessful(t, fname)
 	db := mustLoad(t, fname)
 
 	if err := db.Write(); err != nil {
@@ -170,8 +140,7 @@ func TestWrite(t *testing.T) {
 }
 
 func TestNew(t *testing.T) {
-	fname := fmt.Sprintf("%s/userdb_test-%d", os.TempDir(), os.Getpid())
-	defer os.Remove(fname)
+	fname := t.TempDir() + "/userdb"
 	db1 := New(fname)
 	db1.AddUser("user", "passwd")
 	db1.Write()
@@ -188,7 +157,6 @@ func TestNew(t *testing.T) {
 
 func TestInvalidUsername(t *testing.T) {
 	fname := mustCreateDB(t, "")
-	defer removeIfSuccessful(t, fname)
 	db := mustLoad(t, fname)
 
 	// Names that are invalid.
@@ -227,7 +195,6 @@ func plainPassword(p string) *Password {
 // debugging, but it should be functional for that purpose.
 func TestPlainScheme(t *testing.T) {
 	fname := mustCreateDB(t, "")
-	defer removeIfSuccessful(t, fname)
 	db := mustLoad(t, fname)
 
 	db.db.Users["user"] = plainPassword("pass word")
@@ -248,7 +215,6 @@ func TestPlainScheme(t *testing.T) {
 // Test the denied scheme.
 func TestDeniedScheme(t *testing.T) {
 	fname := mustCreateDB(t, "")
-	defer removeIfSuccessful(t, fname)
 	db := mustLoad(t, fname)
 
 	db.db.Users["user"] = &Password{Scheme: &Password_Denied{}}
@@ -266,7 +232,6 @@ func TestDeniedScheme(t *testing.T) {
 func TestReload(t *testing.T) {
 	content := "users:< key: 'u1' value:< plain:< password: 'pass' >>>"
 	fname := mustCreateDB(t, content)
-	defer removeIfSuccessful(t, fname)
 	db := mustLoad(t, fname)
 
 	// Add a valid line to the file.
@@ -306,7 +271,6 @@ func TestReload(t *testing.T) {
 
 func TestRemoveUser(t *testing.T) {
 	fname := mustCreateDB(t, "")
-	defer removeIfSuccessful(t, fname)
 	db := mustLoad(t, fname)
 
 	if ok := db.RemoveUser("unknown"); ok {
@@ -332,7 +296,6 @@ func TestRemoveUser(t *testing.T) {
 
 func TestExists(t *testing.T) {
 	fname := mustCreateDB(t, "")
-	defer removeIfSuccessful(t, fname)
 	db := mustLoad(t, fname)
 
 	if db.Exists("unknown") {