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/components/inline_oneboxer_spec.rb

@@ -24,7 +24,7 @@ describe InlineOneboxer do
   end
 
   context "caching" do
-    let(:topic) { Fabricate(:topic) }
+    fab!(:topic) { Fabricate(:topic) }
 
     before do
       InlineOneboxer.purge(topic.url)