[Backport] Fix negative subtotal when full discount applied with tax calculation #10790

[ilnytskyi]

Original Pull Request

#14468
Added additional check for discount

Description

Added additional check for full (100%) discount in
app/code/Magento/SalesRule/Model/Utility.php:214
to see if delta (row total minus discount) is negative. In case of negative delta to decrease a discount on delta value.

Fixed Issues (if relevant)

1. Tax rate + 100% discount results in negative grand total #10790 Tax rate + 100% discount results in negative grand total

Manual testing scenarios

1. Create product A with price = 2 762,71
2. Create product B with price = 2 542,37
3. Setup tax rule 18%
4. Create coupon code that adds 100% discount.
5. Add both products to the cart with qty = 2
6. Apply coupon code
7. Total Order should be 0

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)

[m2-assistant]

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[magento-engcom-team]

Hi @rogyar, thank you for the review.
ENGCOM-4718 has been created to process this Pull Request

[VasylShvorak]

✔️ QA passed

[m2-assistant]

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