Mocking or not registry-like object in phpunit tests

I'm facing some difficulties to mock a registry-like object in my tests, this might be because of a wrong design but it always end using $this->at(x) and, IMHO, therefore tightly coupling my test with tested code.

For example, my registry-like object looks like something like this ( remember that its not real code, juste an example ) :

class registrylike{
    public function set($k,$v){$this->$k=$v;return $this;}
    public function get($v){return $this->$v;}
}

let say my to be tested class is :

class main{
    protected $registrylike;

    public function get_ready($registrylike){
        $this->registrylike = $registrylike;
        $this->registrylike->set('k1','v1');
        $this->registrylike->set('k2','v2');
    }
    public function do_something(){
        return sprintf('%s%s',$this->registrylike->get('k1'),$this->registrylike->get('k2'));
    }
}

and my (simple) test is :

class mainTest extends PHPUnit_Framework_TestCase {
    protected $tested;
    public function setUp() {
        $this->tested= new main;
        $registrymock = $this->getMock('registrylike');
        $registrymock->expects($this->at(2))
        ->method('get')
        ->with('k1')
        ->will($this->returnValue('v1'));
        $registrymock->expects($this->at(3))
        ->method('get')
        ->with('k2')
        ->will($this->returnValue('v2'));
        $this->tested->get_ready($registrymock);
    }
    public function test_do_something(){
        $this->AssertEquals($this->tested->do_something(),'v1v2');
   }
}

Is there any way to avoid using $this->at(x)? Should I reconsider this design or use a "real" registrylike instance instead of a mocked one?

Thanks in advance for your answers