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/budgets_spec.rb

@@ -1,11 +1,6 @@
 require "rails_helper"
 
-describe "Admin budgets" do
-  before do
-    admin = create(:administrator)
-    login_as(admin.user)
-  end
-
+describe "Admin budgets", :admin do
   context "Feature flag" do
     before do
       Setting["process.budgets"] = nil