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/models/draft_sequence_spec.rb

@@ -3,7 +3,7 @@
 require 'rails_helper'
 
 describe DraftSequence do
-  let(:user) { Fabricate(:user) }
+  fab!(:user) { Fabricate(:user) }
 
   it 'should produce next sequence for a key' do
     expect(DraftSequence.next!(user, 'test')).to eq 1