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/javascript_cache_spec.rb

@@ -2,7 +2,7 @@
 require 'rails_helper'
 
 RSpec.describe JavascriptCache, type: :model do
-  let(:theme) { Fabricate(:theme) }
+  let!(:theme) { Fabricate(:theme) }
   let(:theme_field) { ThemeField.create!(theme: theme, target_id: 0, name: "header", value: "<a>html</a>") }
 
   describe '#save' do