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/inline_onebox_controller_spec.rb

@@ -20,7 +20,7 @@ describe InlineOneboxController do
     end
 
     context "topic link" do
-      let(:topic) { Fabricate(:topic) }
+      fab!(:topic) { Fabricate(:topic) }
 
       it "returns information for a valid link" do
         get "/inline-onebox.json", params: { urls: [ topic.url ] }