Add controller spec for manifests controller (#4003)

spec/controllers/manifests_controller_spec.rb

+require 'rails_helper'
+
+describe ManifestsController do
+  render_views
+
+  describe 'GET #show' do
+    before do
+      get :show, format: :json
+    end
+
+    it 'assigns @instance_presenter' do
+      expect(assigns(:instance_presenter)).to be_kind_of InstancePresenter
+    end
+
+    it 'returns http success' do
+      expect(response).to have_http_status(:success)
+    end
+  end
+end