MAGETWO-95829: Product positions are incorrect after import

- Fixed to correct zero position during product import process
- Fixed to correct sorting position on admin Category edit page

Author: oksana-kr

app/code/Magento/Catalog/Model/Category/Product/PositionResolver.php

@@ -43,6 +43,8 @@ public function getPositions(int $categoryId): array
             $categoryId
         )->order(
             'ccp.position ' . \Magento\Framework\DB\Select::SQL_ASC
+        )->order(
+            'ccp.product_id ' . \Magento\Framework\DB\Select::SQL_DESC
         );
 
         return array_flip($connection->fetchCol($select));

app/code/Magento/Catalog/Test/Unit/Model/Category/Product/PositionResolverTest.php

@@ -107,7 +107,7 @@ public function testGetPositions()
         $this->select->expects($this->once())
             ->method('where')
             ->willReturnSelf();
-        $this->select->expects($this->once())
+        $this->select->expects($this->exactly(2))
             ->method('order')
             ->willReturnSelf();
         $this->select->expects($this->once())

app/code/Magento/CatalogImportExport/Model/Import/Product.php

@@ -1423,7 +1423,7 @@ protected function _saveProductCategories(array $categoriesData)
                 $delProductId[] = $productId;
 
                 foreach (array_keys($categories) as $categoryId) {
-                    $categoriesIn[] = ['product_id' => $productId, 'category_id' => $categoryId, 'position' => 1];
+                    $categoriesIn[] = ['product_id' => $productId, 'category_id' => $categoryId, 'position' => 0];
                 }
             }
             if (Import::BEHAVIOR_APPEND != $this->getBehavior()) {