feat: add test mode to quizzes and withhold its solutions

- mode, navigation and max_attempts columns with Go-side defaults
- classes.Withheld gates every solution field of the answer feedback
  and the attempt result while a test attempt is open
- a test's quiz detail lists no questions before the attempt starts
- admin fields and lang keys for the three settings

plugin_test.go

@@ -39,8 +39,12 @@ func TestPluginActivates(t *testing.T) {
 	if got := len(p.(pact.HasModels).Models()); got != 5 {
 		t.Errorf("Models = %d, want 5", got)
 	}
-	if len(p.(pact.HasMigrations).Migrations()) == 0 {
-		t.Error("no migrations")
+	migrations := p.(pact.HasMigrations).Migrations()
+	if len(migrations) == 0 {
+		t.Fatal("no migrations")
+	}
+	if got := migrations[len(migrations)-1].ID; got != "20261109120000_add_test_mode_to_quizzes" {
+		t.Errorf("last migration = %s, want 20261109120000_add_test_mode_to_quizzes", got)
 	}
 	if app.Config.Bool("golem15.quizzes.scoring.multiple_partial") {
 		t.Error("scoring.multiple_partial defaults to true, want false")