test: replace tar alias with shell function for macOS compatibility

[rosygmiki]

With the tar alias failing on previous macOS runs, this PR replaces it with the tar() shell function defined in use_gnu_tar_if_available() which uses gtar instead of tar if available.

The previous alias tar=gtar did not propagate inside the create_tar() function, causing it to use BSD tar instead of the required GNU tar.

Closes #83

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.19%. Comparing base (afb00d4) to head (de91302).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
+ Coverage   72.87%   79.19%   +6.31%     
==========================================
  Files          10       10              
  Lines        1412     1115     -297     
==========================================
- Hits         1029      883     -146     
+ Misses        271      134     -137     
+ Partials      112       98      -14     

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[gemini-code-assist]

Code Review

This pull request updates tests/utils.sh to replace the tar alias with a shell function when gtar is available. The review feedback suggests removing the export -f command to avoid unnecessary environment pollution and Bash-specific extensions, as the function is already available within the sourced script's context.

[djach7]

LGTM