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_spec.rb

@@ -2,10 +2,6 @@ require "rails_helper"
 
 describe "Admin" do
   let(:user) { create(:user) }
-  let(:administrator) do
-    create(:administrator, user: user)
-    user
-  end
 
   scenario "Access as regular user is not authorized" do
     login_as(user)
@@ -55,16 +51,14 @@ describe "Admin" do
     expect(page).to have_content "You do not have permission to access this page"
   end
 
-  scenario "Access as administrator is authorized" do
-    login_as(administrator)
+  scenario "Access as administrator is authorized", :admin do
     visit admin_root_path
 
     expect(page).to have_current_path(admin_root_path)
     expect(page).not_to have_content "You do not have permission to access this page"
   end
 
-  scenario "Admin access links" do
-    login_as(administrator)
+  scenario "Admin access links", :admin do
     visit root_path
 
     expect(page).to have_link("Administration")
@@ -73,8 +67,7 @@ describe "Admin" do
     expect(page).to have_link("Management")
   end
 
-  scenario "Admin dashboard" do
-    login_as(administrator)
+  scenario "Admin dashboard", :admin do
     visit root_path
 
     click_link "Administration"
@@ -85,8 +78,7 @@ describe "Admin" do
     expect(page).not_to have_css("#valuation_menu")
   end
 
-  scenario "Admin menu does not hide active elements", :js do
-    login_as(administrator)
+  scenario "Admin menu does not hide active elements", :js, :admin do
     visit admin_budgets_path
 
     within("#admin_menu") do