Refactor Devise integration

- Use scoped users
- Use the named route helpers instead of hardcoded paths

test/integration/admin/login_test.rb

@@ -0,0 +1,27 @@
+require 'test_helper'
+
+class AdminAreaLoginTest < ActionDispatch::IntegrationTest
+  def setup
+    @user = users(:admin)
+  end
+
+  def test_correct_username_and_password
+    visit new_admin_user_session_url
+    fill_in 'admin_user_username', with: @user.username
+    fill_in 'admin_user_password', with: 'testtest'
+    click_button 'Log in'
+
+    assert_text 'Log out'
+    assert_current_path admin_root_path
+  end
+
+  def test_wrong_password
+    visit new_admin_user_session_url
+    fill_in 'admin_user_username', with: @user.username
+    fill_in 'admin_user_password', with: 'wrong'
+    click_button 'Log in'
+
+    assert_text 'Authorization error'
+    assert_current_path new_admin_user_session_path
+  end
+end