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/backups_controller_spec.rb

@@ -3,7 +3,7 @@
 require 'rails_helper'
 
 RSpec.describe Admin::BackupsController do
-  let(:admin) { Fabricate(:admin) }
+  fab!(:admin) { Fabricate(:admin) }
   let(:backup_filename) { "2014-02-10-065935.tar.gz" }
   let(:backup_filename2) { "2014-02-11-065935.tar.gz" }