Navigation API: test same-document back() in an iframe

[zcorpan]

Same-document traversals initiated from an iframe fire the traverse navigate event later in "apply the history step" than top-level traversals do. Check that committed and finished fulfill, that an intercept() handler runs, and that navigatesuccess fires exactly once.

See whatwg/html#12838

[zcorpan]

Claude found this gap while reviewing the spec PR:

On testability: the top-level effect is already covered by return-value/back.html, ordering-and-transition/back-same-document.html, and back-same-document-intercept.html, all of which the current spec text fails and the PR makes pass. The gap I found is iframes. For a same-document traversal of an iframe the traverse event fires at 12.7, and under the current spec the "set the ongoing navigation to null" at 14.10.1 queues an abort before the afterPotentialUnloads task, so back() in an iframe gets navigateerror, finished rejects, and an intercept() handler never runs. back-forward-multiple-frames.html only awaits committed, so that isn't caught.