Enable AvoidCreate cop for spec/lib/sidebars

What does this MR do and why?

Related #414635 (closed)

Follow up on !122198 (comment 1414273472)

Enable AvoidCreate cop for spec/lib/sidebars and ee/spec/lib/sidebars

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spec/lib/sidebars/admin/menus/abuse_reports_menu_spec.rb

@@ -11,7 +11,7 @@
   it_behaves_like 'Admin menu without sub menus', active_routes: { controller: :abuse_reports }
 
   describe '#pill_count' do
-    let_it_be(:user) { build_stubbed(:user, :admin) }
+    let(:user) { build_stubbed(:user, :admin) }
 
     let(:context) { Sidebars::Context.new(current_user: user, container: nil) }