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/budget_phases_spec.rb

@@ -3,12 +3,7 @@ require "rails_helper"
 describe "Admin budget phases" do
   let(:budget) { create(:budget) }
 
-  context "Edit" do
-    before do
-      admin = create(:administrator)
-      login_as(admin.user)
-    end
-
+  context "Edit", :admin do
     scenario "Update phase", :js do
       visit edit_admin_budget_budget_phase_path(budget, budget.current_phase)