add serializers test

spec/serializers/issuable_sidebar_extras_entity_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe IssuableSidebarExtrasEntity do
+  let(:user) { create(:user) }
+  let(:project) { create(:project, :repository) }
+  let(:resource) { create(:issue, project: project) }
+  let(:request) { double('request', current_user: user) }
+
+  subject { described_class.new(resource, request: request).as_json }
+
+  it 'have subscribe attributes' do
+    expect(subject).to include(:participants,
+                               :project_emails_disabled,
+                               :subscribe_disabled_description,
+                               :subscribed,
+                               :assignees)
+  end
+end