Issue #122: CartManager::addLine to protected

- CartManager.php: addLine is now protected

- CartManagerTest: Use purchaseable + addProduct instead of addLine

- Adapt dataProviders for changes from addLine to addProduct:
      AddLine does not run the stock count check like addProduct does so we have
      to change the expected result count to match the product stock count

- AbstractCartManagerTest: Stop using addLine and use addProduct instead

Fix wrong variable