Documentation for setting the current user in test

Ran in to this problem while moving from the built in authentication to multiauth.

The implementation in the Foundation TestCase is here:
https://github.com/laravel/framework/blob/master/src/Illuminate/Foundation/Testing/TestCase.php#L404

Backwards compatibility could be hacked in by adding a #driver() function on MultiManager.php to handle searching by key, but that seems like a bit of a hack just to make testing a bit nicer but I'm open to suggestions. For now I can add some documentation to get people pointed in the right direction.