Improve project iterations tests

Improve tests as a result of reviewer feedback

ee/app/views/layouts/nav/sidebar/_project_iterations_link.html.haml

 - return unless Feature.enabled?(:project_iterations, @project.group, default_enabled: false)
+- return unless @project.feature_available?(:iterations)
 - return unless can?(current_user, :read_iteration, @project)
 
 = nav_link(controller: :iterations) do

ee/spec/controllers/projects/iterations_controller_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Projects::IterationsController do
+  let_it_be(:project) { create(:project) }
+  let_it_be(:user) { create(:user) }
+
+  context 'index' do
+    context 'when iterations license is not available' do
+      before do
+        stub_licensed_features(iterations: false)
+        sign_in(user)
+        get :index, params: { namespace_id: project.namespace, project_id: project }
+      end
+
+      it 'renders 404' do
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+
+    context 'when user is unauthorized' do
+      before do
+        sign_in(user)
+        get :index, params: { namespace_id: project.namespace, project_id: project }
+      end
+
+      it 'renders 404' do
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+
+    context 'when user is authorized' do
+      before do
+        project.add_developer(user)
+        sign_in(user)
+        get :index, params: { namespace_id: project.namespace, project_id: project }
+      end
+
+      it 'renders index successfully' do
+        expect(response).to be_successful
+      end
+    end
+  end
+end