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/requests/admin/dashboard_controller_spec.rb

@@ -14,7 +14,7 @@ describe Admin::DashboardController do
   end
 
   context 'while logged in as an admin' do
-    let(:admin) { Fabricate(:admin) }
+    fab!(:admin) { Fabricate(:admin) }
 
     before do
       sign_in(admin)