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/auth/default_current_user_provider_spec.rb

@@ -368,7 +368,7 @@ describe Auth::DefaultCurrentUserProvider do
   end
 
   describe "#current_user" do
-    let(:user) { Fabricate(:user) }
+    fab!(:user) { Fabricate(:user) }
 
     let(:unhashed_token) do
       new_provider = provider('/')
@@ -605,7 +605,7 @@ describe Auth::DefaultCurrentUserProvider do
   end
 
   context "user api" do
-    let :user do
+    fab! :user do
       Fabricate(:user)
     end