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

@@ -1,8 +1,9 @@
-class FormFixture < Form
+if Rails.env.test?
+  class FormFixture < Form
     attr_accessor :sections, :subsections, :pages, :questions
-end
+  end
 
-class FormFactory
+  class FormFactory
     def initialize(year:, type:)
       @year = year
       @type = type
@@ -22,4 +23,5 @@ class FormFactory
       form.questions = form.pages.flat_map(&:questions)
       form
     end
+  end
 end