Wait for requests before continuing

Otherwise async requests while logging in with admin mode will happen
while factory_bot fixtures are created, causing Rails to lock, and the
spec to hang.

See https://gitlab.com/gitlab-org/gitlab/-/issues/337039

spec/features/admin/admin_runners_spec.rb

@@ -11,6 +11,8 @@ RSpec.describe "Admin Runners" do
     admin = create(:admin)
     sign_in(admin)
     gitlab_enable_admin_mode_sign_in(admin)
+
+    wait_for_requests
   end
 
   describe "Runners page", :js do