Convert admin/settings controller specs to system specs (#31548)

spec/system/admin/settings/registrations_spec.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe 'Admin::Settings::Registrations' do
+  it 'Saves changes to registrations settings' do
+    sign_in admin_user
+    visit admin_settings_registrations_path
+
+    select open_mode_option,
+           from: registrations_mode_field
+
+    click_on submit_button
+
+    expect(page)
+      .to have_content(success_message)
+  end
+
+  def open_mode_option
+    I18n.t('admin.settings.registrations_mode.modes.open')
+  end
+
+  def registrations_mode_field
+    form_label 'form_admin_settings.registrations_mode'
+  end
+end