Intermittent TIMEOUT in /fetch/api/redirect/redirect-keepalive.https.any.html

https://github.com/servo/servo/actions/runs/8847070327/job/24294815667

```
  ▶ TIMEOUT [expected OK] /fetch/api/redirect/redirect-keepalive.https.any.html
  │   ▶ TIMEOUT [expected PASS] [keepalive][iframe][load] mixed content redirect; setting up
  └   └   → Test timed out
```