Blast545/fix unused gcc warnings

[Blast545]

🦟 Bug fix

Fixes gcc warnings appearing in CI: https://build.osrfoundation.org/job/gz_sim-ci-main-jammy-amd64/66/gcc/

Summary

Does not change at all the tests, just voids the variables so the compiler takes them as read and won't emit the warning

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 65.91%. Comparing base (cffd297) to head (e986c4b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## gz-sim8 #2388 +/- ## ======================================== Coverage 65.91% 65.91% ======================================== Files 327 327 Lines 31314 31314 ======================================== Hits 20641 20641 Misses 10673 10673 ```

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[Blast545]

:farmer: Green CI, except on windows, INTEGRATION_mesh_inertia_calculation.test_ran seems to be flaky. FYI @Crola1702

Merging!