app/models/guest_user.rb

@@ -0,0 +1,3 @@
+class GuestUser
+  def registered? = false
+end

app/models/user.rb

@@ -1,3 +1,5 @@
 class User < ApplicationRecord
   has_secure_password
+
+  def registered? = true
 end

test/models/guest_user_test.rb

@@ -0,0 +1,11 @@
+require "test_helper"
+
+class GuestUserTest < ActiveSupport::TestCase
+  setup do
+    @user = GuestUser.new
+  end
+
+  def test_registered_false
+    assert_not @user.registered?
+  end
+end

test/models/user_test.rb

@@ -1,7 +1,11 @@
 require "test_helper"
 
 class UserTest < ActiveSupport::TestCase
-  # test "the truth" do
-  #   assert true
-  # end
+  setup do
+    @user = users(:one)
+  end
+
+  def test_registered_true
+    assert @user.registered?
+  end
 end