Remove usage of assigns in controller specs (#30195)

spec/controllers/admin/instances_controller_spec.rb

@@ -28,12 +28,15 @@ RSpec.describe Admin::InstancesController do
     it 'renders instances' do
       get :index, params: { page: 2 }
 
-      instances = assigns(:instances).to_a
-      expect(instances.size).to eq 1
-      expect(instances[0].domain).to eq 'less.popular'
+      expect(instance_directory_links.size).to eq(1)
+      expect(instance_directory_links.first.text.strip).to match('less.popular')
 
       expect(response).to have_http_status(200)
     end
+
+    def instance_directory_links
+      Nokogiri::Slop(response.body).css('div.directory__tag a')
+    end
   end
 
   describe 'GET #show' do