Fix mocking out-file in describe block

When trying to mock "out-file" in a describe block, I ran across this lovely message:

```
. : The term 'C:\Users\user\AppData\Local\Temp\pester\temp.ps1' is not recognized as the name of a 
cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was 
included, verify that the path is correct and try again.
At C:\path\to\Pester\Functions\It.ps1:71 char:7
+     . $TestDrive\temp.ps1
+       ~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (C:\Users\user...pester\temp.ps1:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException
```

This was fixed by extending the reference to the actual out-file cmdlet so that the mocked version wouldn't be called in the Pester framework.