Intermittent TIMEOUT in `/html/semantics/embedded-content/the-iframe-element/iframe_sandbox_navigate_other_frame_popup.sub.html`

https://github.com/servo/servo/actions/runs/18300096477/job/52106618082
```
Stable unexpected results (1): 
  ▶ TIMEOUT [expected OK] /html/semantics/embedded-content/the-iframe-element/iframe_sandbox_navigate_other_frame_popup.sub.html
  │   ▶ TIMEOUT [expected FAIL] Sandboxed iframe can not navigate other frame's popup
  └   └   → Test timed out
```