add impl of subsite_layout to Iiif::AccessController and spec render

DLC-1177

app/controllers/iiif/access_controller.rb

@@ -25,4 +25,8 @@ def login
       format.html { render action: 'login', layout: 'minimal' }
     end
   end
+
+  def subsite_layout
+    Site::LAYOUT_GALLERY
+  end
 end

spec/features/iiif/access_controller_spec.rb

@@ -0,0 +1,21 @@
+require 'rails_helper'
+
+describe Iiif::AccessController, type: :feature do
+  describe "login" do
+    let(:authorized_user) { FactoryBot.create(:user, is_admin: true) }
+
+    before do
+      Warden.test_mode!
+      login_as authorized_user, scope: :user
+      visit "/iiif/3/login"
+    end
+
+    after do
+      Warden.test_reset!
+    end
+
+    it "shows the success message" do
+      expect(page).to have_text('Authentication successful!')
+    end
+  end
+end