chore(api): Add UTs for validators APIs

[abi87]

Added much needed unit tests around FilterValidators

[codecov]

Codecov Report

Attention: Patch coverage is 46.15385% with 21 lines in your changes missing coverage. Please review.

Project coverage is 32.73%. Comparing base (99ed0c2) to head (41a6c4c).
Report is 3 commits behind head on node-api-validators.

Files with missing lines	Patch %	Lines
node-api/handlers/beacon/types/conversions.go	46.15%	14 Missing and 7 partials ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           node-api-validators    #2519      +/-   ##
=======================================================
+ Coverage                32.04%   32.73%   +0.68%     
=======================================================
  Files                      351      351              
  Lines                    15801    15842      +41     
  Branches                    20       20              
=======================================================
+ Hits                      5064     5186     +122     
+ Misses                   10374    10272     -102     
- Partials                   363      384      +21     

Files with missing lines	Coverage Δ
node-api/handlers/beacon/types/conversions.go	38.15% <46.15%> (+38.15%)	⬆️

... and 4 files with indirect coverage changes

[abi87]

TODO: add property tests to ensure ValidatorToConsensus and ValidatorFromConsensus are dual