Don't require 'turn' from rails/test_help

If 'turn' is in the Gemfile with `:require => nil`, Rails should
respect that.

This supports the case where 'turn' may or may not be required
depending on different developers' preference or environment. E.g., you
require turn for local testing, but not on your CI server by putting
this in `test/test_helper.rb`:

    require 'turn' if ENV['TURN']
Merge pull request #8801 from ktheory/dont_require_turn

Don't require 'turn' from rails/test_help