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/integration/user_api_keys_spec.rb

@@ -4,7 +4,7 @@ require 'rails_helper'
 
 describe 'user api keys integration' do
 
-  let(:user_api_key) { Fabricate(:readonly_user_api_key) }
+  fab!(:user_api_key) { Fabricate(:readonly_user_api_key) }
 
   it 'updates last used time on use' do
     user_api_key.update_columns(last_used_at: 7.days.ago)