## Issue Addressed
Fix a deadlock in the tests that was causing tests on tree-states to run for hours without finishing: https://github.com/sigp/lighthouse/actions/runs/6491194654/job/17628138360.
## Proposed Changes
Avoid using a Mutex under the Rayon `par_iter`. Instead, use an `AtomicUsize`. I've run the new version several times in a loop and it hasn't deadlocked (it was deadlocking consistently on tree-states).
## Additional Info
The same bug exists in unstable and tree-states, but I'm not sure why it was triggering so consistently on the tree-states branch.
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