Right now this stub is bypassing the entire `virt_what` call, which means that the actual behavior isn't being tested. What we need to do here is stub the output of `Facter::Util::Resolution.exec` so that we can pretend the command was run and ensure that the `virt_what` method doesn't return the message. Something like the following should do the trick:

```ruby
it "should not end up in the virtual fact" do
  virt_what_warning = "virt-what: this script must be run as root"
  Facter::Util::Resolution.expects(:exec).with('virt-what 2>/dev/null').returns virt_what_warning
  Facter::Util::Virtual.virt_what.should_not match /^virt-what: /
end
```

Try updating this pull request with something like the above test and hopefully the tests will pass.