tests: add normalize_attribute helper method.

Add a custom Shoulda matcher for testing that a model correctly normalizes an attribute.

Usage:

    subject { build(:wiki_page) }
    should normalize_attribute(:title).from(" Azur  Lane ").to("azur_lane")

test/test_helper.rb

@@ -27,6 +27,7 @@ class ActiveSupport::TestCase
   include IqdbTestHelper
   include UploadTestHelper
   extend StripeTestHelper
+  extend NormalizeAttributeHelper
 
   mock_post_version_service!
   mock_pool_version_service!