fix path to helper_functions

[dpo]

closes #238

[codecov]

Codecov Report

Merging #241 into master will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #241      +/-   ##
==========================================
- Coverage   83.55%   83.53%   -0.03%     
==========================================
  Files           5        5              
  Lines         590      589       -1     
==========================================
- Hits          493      492       -1     
  Misses         97       97              

Impacted Files	Coverage Δ
src/CUTEst.jl	79.14% <100.00%> (-0.13%)	⬇️

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[abelsiqueira]

Thanks!