test: fix invalid test variable type

[aloisklink]

My fault, I accidentally wrote const instead of char, and C implicitly assumed that it was an int pointer.

Maybe it's worth setting -Werror in our code so that all C warnings become errors.

[codecov]

Codecov Report

Merging #266 (7ba1c56) into main (149c1a7) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 7ba1c56 differs from pull request most recent head 826b55f. Consider uploading reports for the commit 826b55f to get more accurate results

@@           Coverage Diff           @@
##             main     #266   +/-   ##
=======================================
  Coverage   48.51%   48.51%           
=======================================
  Files         113      113           
  Lines       17887    17887           
=======================================
  Hits         8678     8678           
  Misses       9209     9209           

Impacted Files	Coverage Δ
tests/utils/test_os.c	97.60% <100.00%> (ø)
src/utils/ifaceu.c	100.00% <0.00%> (ø)

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[mereacre]

I added once (a year or so ago) the -Werror. There was a bunch of errors. But we should add it again. I think this time there will be less errors.