[Backport] Tier price on configurable product sorting sometimes wrong

[amol2jcommerce]

Original Pull Request

magento#20407

Description (*)

Tier prices on configurable product have sorting that differ then in back office.

Fixed Issues (if relevant)

• Tier price on configurable product sorting sometimes wrong magento/magento2#12194: Tier price on configurable product sorting sometimes wrong

Manual testing scenarios (*)

1. edit option assigned to configurable product
2. add tier prices to option:

• all websites ; all groups ; qty 4; price 1
• all websites ; all groups ; qty 3; price 2
• all websites ; all groups ; qty 2; price 3

3. save option
4. visit configurable product detail page
   Tier Prices on frontend should be in displayed in same order as in back office

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)