chore: Move the validation func from helpers to ramping vus

[codebien]

What?

I had it on a draft for a long time and I think is still worth considering the low effort. It amends what we did on #3470 moving the function closer to where they are used.

Why?

The logic implemented is specific for ramping-vus and it shouldn't be reusable for other executors. Then it's better to have it in the specific file instead of the generic helpers.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.58%. Comparing base (b217bbf) to head (c4d7837).

❗ Current head c4d7837 differs from pull request most recent head b0da697. Consider uploading reports for the commit b0da697 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3616   +/-   ##
=======================================
  Coverage   73.57%   73.58%           
=======================================
  Files         277      277           
  Lines       20244    20244           
=======================================
+ Hits        14895    14896    +1     
+ Misses       4401     4400    -1     
  Partials      948      948           

Flag	Coverage Δ
macos	?
ubuntu	73.49% <100.00%> (-0.03%)	⬇️
windows	73.43% <100.00%> (+0.01%)	⬆️

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