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/stats_spec.rb

@@ -1,9 +1,7 @@
 require "rails_helper"
 
-describe "Stats" do
+describe "Stats", :admin do
   before do
-    admin = create(:administrator)
-    login_as(admin.user)
     visit root_path
   end