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/dashboard/administrator_tasks_spec.rb

@@ -1,12 +1,6 @@
 require "rails_helper"
 
-describe "Admin administrator tasks" do
-  let(:admin) { create :administrator }
-
-  before do
-    login_as(admin.user)
-  end
-
+describe "Admin administrator tasks", :admin do
   context "when visiting index" do
     context "and no pending tasks" do
       before do