DEV: Prefabrication (test optimization) (#7414)

* Introduced fab!, a helper that creates database state for a group

It's almost identical to let_it_be, except:

 1. It creates a new object for each test by default,
 2. You can disable it using PREFABRICATION=0

spec/requests/admin/permalinks_controller_spec.rb

@@ -8,7 +8,7 @@ describe Admin::PermalinksController do
     expect(Admin::PermalinksController < Admin::AdminController).to eq(true)
   end
 
-  let(:admin) { Fabricate(:admin) }
+  fab!(:admin) { Fabricate(:admin) }
 
   before do
     sign_in(admin)