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/mailers/rejection_mailer_spec.rb

@@ -7,7 +7,7 @@ describe RejectionMailer do
   describe "send_rejection" do
 
     context 'sends rejection email' do
-      let (:user) { Fabricate(:user) }
+      fab! (:user) { Fabricate(:user) }
       let (:template_args) { { former_title: "Mail Subject", destination: user.email, site_name: SiteSetting.title } }
       let (:reject_mail) { RejectionMailer.send_rejection("email_reject_topic_not_found", user.email, template_args) }