Use an HTML5 parser in test helpers

I think this doesn't affect us because we use RSpec instead of Rails
test classes. In any case, if it ever affects us, we'll get notified
when a test fails.

config/initializers/new_framework_defaults_7_1.rb

@@ -256,4 +256,4 @@ Rails.application.config.action_dispatch.debug_exception_log_level = :error
 #
 # In previous versions of Rails, these test helpers always used an HTML4 parser.
 #++
-# Rails.application.config.dom_testing_default_html_version = :html5
+Rails.application.config.dom_testing_default_html_version = :html5