Increase unit test coverage for ValidatorList

[Bronek]

High Level Overview of Change

Just another mildly interesting test case

Type of Change

• 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 not work as expected)
• Refactor (non-breaking change that only restructures code)
• Performance (increase or change in throughput and/or latency)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.2%. Comparing base (2f432e8) to head (a4be0b5).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #5116   +/-   ##
=======================================
  Coverage     76.2%   76.2%           
=======================================
  Files          760     760           
  Lines        61489   61489           
  Branches      8115    8111    -4     
=======================================
+ Hits         46832   46842   +10     
+ Misses       14657   14647   -10     

see 3 files with indirect coverage changes

[Bronek]

Folded into #5112