Intermittent FAIL in `/dom/nodes/moveBefore/iframe-document-preserve.window.html`

Seen here: https://github.com/servo/servo/actions/runs/22921982816

```
OK /dom/nodes/moveBefore/iframe-document-preserve.window.html

    FAIL [expected PASS] subtest: moveBefore(): cross-origin iframe is preserved: remove self

    assert_equals: iframe does not fire a second load event expected 1 but got 0
```
cc @lukewarlow 