EDIT: As pointed by @DavidBarker in his comment to the OP question
The Illuminate\Support\Facades\Response class doesn't actually extend Illuminate\Support\Facades\Facade so doesnt have the shouldRecieve() method. You need to test the response of this route after calling it in a test.
So if you want to test your download functionality, you can try checking the response for errors with:
$this->assertTrue(preg_match('/(error|notice)/i', $response) === false);