Magento 2 – Fix Fatal Error ‘Call to Undefined Function’ with __() in Unit Test

The problem seems to be that ProductExtension is not generated (this is auto-generated class).

If running Magento in production or default mode, code generation on-fly is disabled and you need to run compiler after installation, execute <project_root>/bin/magento setup:di:compile as described in Magento dev docs.

If running Magento in developer mode, make sure var/generation directory is writable.

The problem is that your constructor does not match the parent class constructor.

To fix that you need to update your constructor:

public function __construct(
    \Magento\Framework\View\Element\Template\Context $context,
    \Magento\Customer\Model\Session $customerSession,  
    \Magento\Framework\ObjectManagerInterface $objectManager,
    array $data = []
 ) {
    parent::__construct($context, $data);
    $this->customerSession = $customerSession;
    $this->_objectManager = $objectManager;
  }

Don't forget to flush the var/cache and var/generation after your changes.