We're seeing some strange behaviour with, I think, LOADPATH. I recently added and apt_key type and provider to puppetlabs-apt and we had to patch open-uri. The code for that was loaded from puppet_x/apt_key/patch_openuri and all your tests passed.
However, people that have modules that used puppetlabs-apt suddenly had their tests blow up on them because the require puppet_x/apt_key/patch_openuri wasn't working. We ended up swapping to a File.expand_path based solution but that's nasty.
The question, what's happening with LOADPATH and why isn't the puppet_x stuff kept on it?
This is basically the issue
MODULES-520 worked around.