MAGETWO-48227: Fix API/Integration tests

- Fixed tests

Author: Gurzhyi, Andrii

app/code/Magento/ConfigurableProduct/Model/Product/ReadHandler.php

@@ -97,7 +97,7 @@ private function getLinkedProducts(ProductInterface $product)
         $metadata = $this->metadataPool->getMetadata(ProductInterface::class);
         /** @var Configurable $typeInstance */
         $typeInstance = $product->getTypeInstance();
-        $childrenIds = $typeInstance->getChildrenIds($product->getData($metadata->getLinkField()));
+        $childrenIds = $typeInstance->getChildrenIds($product->getId());
 
         if (isset($childrenIds[0])) {
             return $childrenIds[0];

dev/tests/integration/testsuite/Magento/ConfigurableProduct/Model/Product/Type/Configurable/PriceTest.php

@@ -117,9 +117,8 @@ public function testGetFinalPriceWithCustomOption()
 
         /** @var \Magento\Catalog\Api\ProductRepositoryInterface $productRepository */
         $productRepository = $this->objectManager->get(ProductRepositoryInterface::class);
-        $productRepository->save($product);
+        $product = $productRepository->save($product);
 
-        $product = $this->getProduct(1);
         $optionId = $product->getOptions()[0]->getId();
         $product->addCustomOption(AbstractType::OPTION_PREFIX . $optionId, 'text');
         $product->addCustomOption('option_ids', $optionId);