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/services/topic_status_updater_spec.rb

@@ -8,8 +8,8 @@ require_dependency 'post_destroyer'
 
 describe TopicStatusUpdater do
 
-  let(:user) { Fabricate(:user) }
-  let(:admin) { Fabricate(:admin) }
+  fab!(:user) { Fabricate(:user) }
+  fab!(:admin) { Fabricate(:admin) }
 
   it "avoids notifying on automatically closed topics" do
     # TODO: TopicStatusUpdater should suppress message bus updates from the users it "pretends to read"