Updated powershell functional tests to work with changes to Chef::Util::Powershell:Cmdlet

@adamedx This makes the tests pass. Was not sure about the extra tests for the invalid cmdlet. It did not seem relevant anymore 
Missed this comment before merge. I think I wasn't aware of `all_plugins` a few months ago when I wrote this (this is ported from the prototype). We'll fix this, thank you @sersut!