Avoid frequent intermittent failure in tests.
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#11613 (github issue number if applicable).
- [X] These changes do not require tests because they address a frequent nondeterministic panic that occurs in other tests.
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/11707)
<!-- Reviewable:end -->