Intermittent FAIL in `/paint-timing/with-lcp/fcp-document-opacity-image.html`

Seen in https://github.com/servo/servo/actions/runs/22778607422:

```
FAIL [expected PASS] subtest: Test that FCP after opacity change is not a larger value than LCP

assert_equals: First contentful paint marked too early.  expected 0 but got 1
```