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/jobs/truncate_user_flag_stats_spec.rb

@@ -3,8 +3,8 @@
 require 'rails_helper'
 
 describe Jobs::TruncateUserFlagStats do
-  let(:user) { Fabricate(:user) }
-  let(:other_user) { Fabricate(:user) }
+  fab!(:user) { Fabricate(:user) }
+  fab!(:other_user) { Fabricate(:user) }
 
   before do
     # We might make this a site setting eventually