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/valuator_groups_spec.rb

@@ -1,12 +1,6 @@
 require "rails_helper"
 
-describe "Valuator groups" do
-  let(:admin) { create(:administrator).user }
-
-  before do
-    login_as(admin)
-  end
-
+describe "Valuator groups", :admin do
   scenario "Index" do
     group1 = create(:valuator_group)
     group2 = create(:valuator_group)