Intermittent PASS in /css/css-cascade/layer-font-face-override.html

https://github.com/servo/servo/actions/runs/13815734246/job/38648826026

```
 Stable unexpected results (1): 
  ▶ OK /css/css-cascade/layer-font-face-override.html
  │   ▶ PASS [expected FAIL] @font-face override update with appended sheet 1
  └   └ PASS [expected FAIL] @font-face override update with appended sheet 2
```