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/anonymous_shadow_creator_spec.rb

@@ -12,7 +12,7 @@ describe AnonymousShadowCreator do
 
     before { SiteSetting.allow_anonymous_posting = true }
 
-    let(:user) { Fabricate(:user, trust_level: 3) }
+    fab!(:user) { Fabricate(:user, trust_level: 3) }
 
     it "returns no shadow if trust level is not met" do
       expect(AnonymousShadowCreator.get(Fabricate.build(:user, trust_level: 0))).to eq(nil)