Fix for Issue-13556 - Sorting in Product Listing via Quantity not work

[nuzil]

This Pull Request related to issue: #13556

Problem

If you edit product attribute 'quantity_and_stock_status' and set "Used for Sorting in Product Listing" to "Yes" sort option will be displayed on category products page but sorting will not work.

Reason

1. Attribute 'quantity_and_stock_status' has source model, and for sorting magento calls addValueSortToCollection method of source model.
2. However source model of this attribute ("Magento\CatalogInventory\Model\Source\Stock") does not have its own method "addValueSortToCollection" only inherits parent one which just do nothing (return $this).

Fix

We added to source model method which contain logic to join to collection (which is always product collection) cataloginventory_stock_item table and add order by qty field.

Manual testing scenarios

Testing scenarios can be found in issue
#13556

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[sidolov]

Changes covered with unit test