Remove final remaining assigns usage in controller specs (#33866)

spec/requests/admin/instances_spec.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'Admin Instances' do
+  describe 'GET /admin/instances/:id' do
+    context 'with an unknown domain' do
+      before { sign_in Fabricate(:admin_user) }
+
+      it 'returns http success' do
+        get admin_instance_path(id: 'unknown.example')
+
+        expect(response)
+          .to have_http_status(200)
+      end
+    end
+  end
+end