test: Handle unknown validator function name

[anishkny]

Description

Added a test case to ensure passing unknown validator function name is handled gracefully.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Added test - no change to code

  Related Issue

Motivation and Context

I wanted to improve code coverage to 100% (from 99.6%)

How Has This Been Tested?

Screenshots (if appropriate):

Checklist:

• [ ] I have updated the documentation (if required).
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.
• [ ] I added a picture of a cute animal cause it's fun

[anishkny]

Coverage bumped to 100%

[lirantal]

Look at that beauty:

Thank you @anishkny, greta job! 👏