Add a test for Settings::Extend#settings (#5720)

spec/lib/settings/extend_spec.rb

+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe Settings::Extend do
+  class User
+    include Settings::Extend
+  end
+
+  describe '#settings' do
+    it 'sets @settings as an instance of Settings::ScopedSettings' do
+      user = Fabricate(:user)
+      expect(user.settings).to be_kind_of Settings::ScopedSettings
+    end
+  end
+end