Fixes flaky test found in nightly run:
https://github.com/fleetdm/fleet/actions/runs/30330273165.
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## Summary by CodeRabbit
* **Tests**
* Improved execution checks to tolerate transient busy-file errors
during retries.
* Strengthened validation for both healthy and corrupted binaries,
ensuring tests report the intended result.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Lucas Manuel Rodriguez
1f2ca668f8