Laravel 5 Model mocking

I have seen a few posts in various places about this and they all seem to have a similar answer. However for the life of me I cannot get the Mockery object working properly.

The Attribute model looks like this

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Attribute extends Model {

    public function test()
    {
        return (new \App\Models\Value())->hello();
    }
}

The Value model like this

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Value extends Model
{
    public function hello()
    {
        return 'goodbye';
    }
}

The PHPUnit test looks like this

use App\Models\Attribute;

class AttributeModelTest extends TestCase
{
    public function testThing()
    {
        $mock = Mockery::mock('\App\Models\Value');

        $mock->shouldReceive('hello')
           ->once()
           ->andReturn('hello');

        $this->app->instance('\App\Models\Value', $mock);

       $a = new \App\Models\Attribute();
       $return = $a->test();
       var_dump($return);
    }
}

PHPUnit outputs 'goodbye', where I though that I am telling it to return 'hello' in the mock and it doesn't. Any ideas what I might be doing wrong?