Intermittent TIMEOUT in /html/semantics/embedded-content/the-iframe-element/sandbox-top-navigation-user-activation-sticky.tentative.sub.window.html

https://github.com/servo/servo/actions/runs/8847070327/job/24294821136

```
  ▶ TIMEOUT [expected OK] /html/semantics/embedded-content/the-iframe-element/sandbox-top-navigation-user-activation-sticky.tentative.sub.window.html
  │   ▶ TIMEOUT [expected FAIL] Allow top with user activation + user activation
  └   └   → Test timed out
```