Deprecate / remove / update Vagrant::TestHelpers

Hi,

While trying to upgrade some old middleware / action specs for a plugin I found out that [Vagrant::TestHelpers](https://github.com/mitchellh/vagrant/blob/master/lib/vagrant/test_helpers.rb) does not work with vagrant >= 1.1. It looks like it is calling [some](https://github.com/mitchellh/vagrant/blob/master/lib/vagrant/test_helpers.rb#L50) [old](https://github.com/mitchellh/vagrant/blob/master/lib/vagrant/test_helpers.rb#L83) `Vagrant::Environment` methods which does not exist anymore.

I thought of sending a patch to fix that but after [looking at one of the builtin actions test](https://github.com/mitchellh/vagrant/blob/master/test/unit/vagrant/action/builtin/confirm_test.rb) it seems that things are easier than I thought. I also "grepped" for usages on Vagrant itself and couldn't find any.

Is it worth upgrading its code? I have a "half baked" fix for that around here in case you think it is :-)