Magento – magento 2 doesn’t copying product picture in tmp folder

I've managed to solve my problem by updating my MassConfigurable::assingProducts method in the following way:

private function assingProducts($productId, $products, $attributes) {
    $pr = $this->productFactory->create();
    $pr->load($productId);
    $pr->setTypeId('configurable');

    /** Associates selected Simple Products to newly created Configurable Product */
    $pr->setAssociatedProductIds($products);
    $pr->getTypeInstance()->setUsedProductAttributeIds($attributes,  $pr); //attributes IDs of product's attributes in my store
    $configurableAttributesData =  $pr->getTypeInstance()->getConfigurableAttributesAsArray($pr);
    $pr->setCanSaveConfigurableAttributes(true);
    $pr->setConfigurableAttributesData($configurableAttributesData);
    $pr->save();

    /** In the following bit of code, the Configurable Product gets duplicated and then it gets deleted.
     *  This leaves the duplicated product as the original one.
     */

    /** @var \Magento\Catalog\Model\Product $duplicate */
    $duplicate = $this->productFactory->create();
    $duplicate->setData($pr->getData());
    $duplicate->setStatus(\Magento\Catalog\Model\Product\Attribute\Source\Status::STATUS_DISABLED);
    $duplicate->setSku($pr->getSku());
    $duplicate->setUrlKey($pr->getUrlKey());
    $duplicate->setId($pr->getId());
    $duplicate->setStoreId(\Magento\Store\Model\Store::DEFAULT_STORE_ID);
    $pr->delete();
    $duplicate->save();
}

What i'm doing is that i load the Configurable Product that i created in createConfigurable() method, assing the simple products to it and then i save it. After that, i duplicate the Configurable Product, delete the original configurable product and save the duplicate in replace of the original.

I'm doing this because i discovered that the problem lies in updating a product programmatically (loading the product, adding or changing some of its attributes and saving it again). After updating the product this way, the product cannot be edited in Admin Panel. For the previous reason i have created another question because the problem exposed here is related but not caused directly by how the configurable product is created. This is the link of the question: How to correctly update product programmatically - Magento 2

In Magento 2, the root folder is not by default accessible. You need to put your testimg folder in pub/media to make it work. Move your folder to pub/media and update your code like below.

Here you are using an external php file, so you can do it like below.

$obj = $bootstrap->getObjectManager();
$dir = $obj->get('Magento\Framework\App\Filesystem\DirectoryList');

$_product->addImageToMediaGallery(
    $dir->getPath('media').'/testimg/test.jpg',
    null,
    false,
    false
);
$_product->save();