I tried to test an observer which should kill the form_key on the add to cart action. To do this I wanted to put a product into the cart and make sure it was added, e.g. by testing that checkout_cart_add_product_complete was dispatched.

But my test responded with:

Failed asserting that request route matches expected one.

The problem is, that magento tries in \Mage_Core_Controller_Varien_Action::preDispatch to make sure, that it is allowed to set cookies. It is reading the cookie $cookies = Mage::getSingleton('core/cookie')->get(); and checked whether there is something in it: if (empty($cookies)). In $cookies should be at least the frontend session id. To make sure, that core/cookie returns an non-empty array, we mock it: $sessionId = 'sessionId1234';
$coreCookieMock =$this->getModelMock('core/cookie', ['get'], false, [], [], false);
$coreCookieMock->method('get')->willReturn(['frontend' =>$sessionId]);
$this->replaceByMock('singleton', 'core/cookie',$coreCookieMock);