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/user_authenticator_spec.rb

@@ -21,7 +21,7 @@ end
 
 describe UserAuthenticator do
   context "#finish" do
-    let(:group) { Fabricate(:group, automatic_membership_email_domains: "discourse.org") }
+    fab!(:group) { Fabricate(:group, automatic_membership_email_domains: "discourse.org") }
 
     before do
       SiteSetting.enable_github_logins = true