string literals; why not.

test/controllers/accounts_controller_test.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'test_helper'
 
 class AccountsControllerTest < ActionDispatch::IntegrationTest

test/controllers/auth_controller_test.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'test_helper'
 
 class AuthControllerTest < ActionDispatch::IntegrationTest

test/models/account_test.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'test_helper'
 
 class AccountTest < ActiveSupport::TestCase

test/services/app_config_test.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'test_helper'
 
 class AppConfigTest < ActiveSupport::TestCase

test/services/crypt_serializer_test.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'test_helper'
 
 class CryptSerializerTest < ActiveSupport::TestCase

test/test_helper.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 ENV['RAILS_ENV'] ||= 'test'
 
 # https://github.com/colszowka/simplecov

test/test_helpers/test_auth_helper.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module TestAuthHelper
   ## Monkey patch AuthController, because...auth.
   AuthController.class_eval do