Load test fixtures where data are needed

Without this, some tests here were not actually testing anything.

activerecord/test/cases/relation/delegation_test.rb

@@ -61,6 +61,8 @@ module ActiveRecord
   end
 
   class DelegationRelationTest < DelegationTest
+    fixtures :comments
+
     def target
       Comment.where.not(body: nil)
     end