Simplify tests requiring admin login

We were repeating the same code over and over (with a few variants) to
setup tests which require an administrator. We can use a tag and
simplify the code.

spec/system/admin/admin_notifications_spec.rb

@@ -1,10 +1,7 @@
 require "rails_helper"
 
-describe "Admin Notifications" do
-  before do
-    create(:budget)
-    login_as(create(:administrator).user)
-  end
+describe "Admin Notifications", :admin do
+  before { create(:budget) }
 
   context "Show" do
     scenario "Valid Admin Notification" do