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/session_controller_spec.rb

@@ -1345,7 +1345,7 @@ RSpec.describe SessionController do
     end
 
     context 'for an existing username' do
-      let(:user) { Fabricate(:user) }
+      fab!(:user) { Fabricate(:user) }
 
       context 'local login is disabled' do
         before do