Add spec for updating reserved usernames setting

spec/features/admin/settings_spec.rb

@@ -0,0 +1,21 @@
+require 'rails_helper'
+
+RSpec.describe 'Admin/global settings', type: :feature do
+  let(:user) { create :user }
+
+  before do
+    allow(Devise::LDAP::Adapter).to receive(:get_ldap_param)
+      .with(user.cn, :admin).and_return(["true"])
+
+    login_as user, :scope => :user
+  end
+
+  scenario 'Update reserved usernames' do
+    visit admin_settings_registrations_path
+    expect(Setting.reserved_usernames).not_to include(['Kosmos', 'Kredits'])
+
+    fill_in 'Reserved usernames', with: "Kosmos\nKredits"
+    click_button "Save"
+    expect(Setting.reserved_usernames).to eq(['Kosmos', 'Kredits'])
+  end
+end