Add role-specific user fabricators (#33437)

spec/controllers/admin/instances_controller_spec.rb

@@ -5,7 +5,7 @@ require 'rails_helper'
 RSpec.describe Admin::InstancesController do
   render_views
 
-  let(:current_user) { Fabricate(:user, role: UserRole.find_by(name: 'Admin')) }
+  let(:current_user) { Fabricate(:admin_user) }
 
   let!(:account_popular_main) { Fabricate(:account, domain: 'popular') }