Load FormFixture only in tests (#1643)

It caused superclass mismatch errors. We can stop seeing this bug in local development by defining the class only in test environments

spec/factories/form.rb

@@ -1,3 +1,4 @@
+if Rails.env.test?
   class FormFixture < Form
     attr_accessor :sections, :subsections, :pages, :questions
   end
@@ -23,3 +24,4 @@ class FormFactory
       form
     end
   end
+end