tests: fix sign-conversion warning in helpers_test

[igchor]

This change is 

[lukaszstolarczuk]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @igchor)

[codecov]

Codecov Report

Merging #1072 (b966987) into master (a2cb574) will decrease coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1072      +/-   ##
==========================================
- Coverage   94.23%   94.02%   -0.22%     
==========================================
  Files          48       48              
  Lines        4751     4751              
==========================================
- Hits         4477     4467      -10     
- Misses        274      284      +10     

Flag	Coverage Δ
tests_clang_debug_cpp17	93.48% <ø> (-0.18%)	⬇️
tests_gcc_debug	91.71% <ø> (-0.41%)	⬇️

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Impacted Files	Coverage Δ
include/libpmemobj++/condition_variable.hpp	61.90% <0.00%> (-16.67%)	⬇️
...ude/libpmemobj++/container/concurrent_hash_map.hpp	94.35% <0.00%> (-0.48%)	⬇️

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